72 results on '"Per-Åke Malmqvist"'
Search Results
2. Parallelization of a multiconfigurational perturbation theory.
- Author
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Steven Vancoillie, Mickaël G. Delcey, Roland Lindh, Victor Vysotskiy, Per-åke Malmqvist, and Valera Veryazov
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- 2013
- Full Text
- View/download PDF
3. MOLCAS 7: The Next Generation.
- Author
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Francesco Aquilante, Luca De Vico, Nicolas Ferré, Giovanni Ghigo, Per-åke Malmqvist, Pavel Neogrády, Thomas Bondo Pedersen, Michal Pitonák, Markus Reiher, Björn Roos, Luis Serrano-Andrés, Miroslav Urban, Valera Veryazov, and Roland Lindh
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- 2010
- Full Text
- View/download PDF
4. Non-radiative decay and fragmentation in water molecules after 1a
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Anna, Sankari, Christian, Stråhlman, Rami, Sankari, Leena, Partanen, Joakim, Laksman, J Antti, Kettunen, Ignacio Fdez, Galván, Roland, Lindh, Per-Åke, Malmqvist, and Stacey L, Sorensen
- Abstract
In this paper, we examine decay and fragmentation of core-excited and core-ionized water molecules combining quantum chemical calculations and electron-energy-resolved electron-ion coincidence spectroscopy. The experimental technique allows us to connect electronic decay from core-excited states, electronic transitions between ionic states, and dissociation of the molecular ion. To this end, we calculate the minimum energy dissociation path of the core-excited molecule and the potential energy surfaces of the molecular ion. Our measurements highlight the role of ultra-fast nuclear motion in the 1a
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- 2020
5. Non-radiative decay and fragmentation in water molecules after 1a1-14a1 excitation and core ionization studied by electron-energy-resolved electron–ion coincidence spectroscopy
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Leena Partanen, Per-Åke Malmqvist, Joakim Laksman, J. Antti Kettunen, Anna Sankari, Roland Lindh, Christian Stråhlman, Rami Sankari, Ignacio Fernández Galván, Stacey L. Sorensen, Tampere University, and Physics
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Fysikalisk kemi ,Materials science ,010304 chemical physics ,Atom and Molecular Physics and Optics ,Polyatomic ion ,General Physics and Astronomy ,010402 general chemistry ,114 Physical sciences ,01 natural sciences ,Molecular physics ,Physical Chemistry ,Dissociation (chemistry) ,0104 chemical sciences ,Ion ,Dication ,Fragmentation (mass spectrometry) ,Ionization ,0103 physical sciences ,Teoretisk kemi ,Molecule ,Atom- och molekylfysik och optik ,Physical and Theoretical Chemistry ,Spectroscopy ,Theoretical Chemistry - Abstract
In this paper, we examine decay and fragmentation of core-excited and core-ionized water molecules combining quantum chemical calculations and electron-energy-resolved electron–ion coincidence spectroscopy. The experimental technique allows us to connect electronic decay from core-excited states, electronic transitions between ionic states, and dissociation of the molecular ion. To this end, we calculate the minimum energy dissociation path of the core-excited molecule and the potential energy surfaces of the molecular ion. Our measurements highlight the role of ultra-fast nuclear motion in the 1a1−14a1" role="presentation" style="display: inline; line-height: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">1a−114a11a1−14a1 core-excited molecule in the production of fragment ions. OH+ fragments dominate for spectator Auger decay. Complete atomization after sequential fragmentation is also evident through detection of slow H+ fragments. Additional measurements of the non-resonant Auger decay of the core-ionized molecule (1a1−1" role="presentation" style="display: inline; line-height: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">1a−111a1−1) to the lower-energy dication states show that the formation of the OH+ + H+ ion pair dominates, whereas sequential fragmentation OH+ + H+ → O + H+ + H+ is observed for transitions to higher dication states, supporting previous theoretical investigations.
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- 2020
6. Modern quantum chemistry with [Open]Molcas
- Author
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Dumitru-Claudiu Sergentu, Leon Freitag, Quan Manh Phung, Ernst D. Larsson, Liviu F. Chibotaru, Francesco Segatta, Per-Åke Malmqvist, Saumik Sen, Javier Segarra-Martí, Irene Conti, Marco Garavelli, Liviu Ungur, Artur Nenov, Alberto Baiardi, Morgane Vacher, Francesco Aquilante, Jesper Norell, Christopher J. Stein, Luis Seijo, Thomas Bondo Pedersen, Kristine Pierloot, Stefano Battaglia, Jochen Autschbach, Massimo Olivucci, Roland Lindh, Nicolas Ferré, Stefan Knecht, Ignacio Fernández Galván, Luca De Vico, Xuejun Gong, Igor Schapiro, Markus Reiher, Michael Odelius, Marcus Lundberg, Veniamin Borin, Mickaël G. Delcey, Laura Pedraza-González, Valera Veryazov, Alessio Valentini, Theory and Simulation of Materials (THEOS) and National Centre for Computational Design and Discovery of Novel Materials, University at Buffalo [SUNY] (SUNY Buffalo), State University of New York (SUNY), Laboratory of Physical Chemistry [ETH Zürich] (LPC), Department of Chemistry and Applied Biosciences [ETH Zürich] (D-CHAB), Eidgenössische Technische Hochschule - Swiss Federal Institute of Technology [Zürich] (ETH Zürich)- Eidgenössische Technische Hochschule - Swiss Federal Institute of Technology [Zürich] (ETH Zürich), Laboratoire de Chimie et Physique Quantiques (LCPQ), Institut de Recherche sur les Systèmes Atomiques et Moléculaires Complexes (IRSAMC), Institut National des Sciences Appliquées - Toulouse (INSA Toulouse), Institut National des Sciences Appliquées (INSA)-Institut National des Sciences Appliquées (INSA)-Université Toulouse III - Paul Sabatier (UT3), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Centre National de la Recherche Scientifique (CNRS)-Institut National des Sciences Appliquées - Toulouse (INSA Toulouse), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Centre National de la Recherche Scientifique (CNRS)-Institut de Chimie du CNRS (INC), The Hebrew University of Jerusalem (HUJ), Institute for Nanoscale Physics and Chemistry (INPAC), Université Catholique de Louvain = Catholic University of Louvain (UCL), Dipartimento di Chimica Industriale 'Toso Montanari', ALMA MATER STUDIORUM-Universitàdi Bologna, Università degli Studi di Siena = University of Siena (UNISI), Uppsala University, Angström Laboratory, Institut de Chimie Radicalaire (ICR), Aix Marseille Université (AMU)-Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS), University of Vienna [Vienna], Dipartimento di Chimica 'G. Ciamician', Alma Mater Studiorum Università di Bologna [Bologna] (UNIBO), National University of Singapore (NUS), Laboratorium für Physikalische Chemie (ETH-LPC), Eidgenössische Technische Hochschule - Swiss Federal Institute of Technology [Zürich] (ETH Zürich), Lund University [Lund], Department of Chemistry-Angstrom, the Theoretical Chemistry Programme, Division of Theoretical Chemistry, AlbaNova University Center (ALBANOVA), Stockholm University, Department of Physics [Stockholm], Dipartimento di Chimica, Department of Biochemistry and Molecular Biology, University of Southern Denmark (SDU), Nagoya University, Catholic University of Leuven - Katholieke Universiteit Leuven (KU Leuven), Bowling Green State University (BGSU), Laboratoire de Chimie - UMR5182 (LC), Centre National de la Recherche Scientifique (CNRS)-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Université de Lyon-École normale supérieure - Lyon (ENS Lyon)-Institut de Chimie du CNRS (INC), Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Científicas [Madrid] (CSIC), Division of Quantum and Chemistry, Department of Chemistry [Imperial College London], Imperial College London, Dipartimento di Produzioni Animali, Università della Tuscia, Aquilante F., Autschbach J., Baiardi A., Battaglia S., Borin V.A., Chibotaru L.F., Conti I., De Vico L., Delcey M., Galvan I.F., Ferre N., Freitag L., Garavelli M., Gong X., Knecht S., Larsson E.D., Lindh R., Lundberg M., Malmqvist P.A., Nenov A., Norell J., Odelius M., Olivucci M., Pedersen T.B., Pedraza-Gonzalez L., Phung Q.M., Pierloot K., Reiher M., Schapiro I., Segarra-Marti J., Segatta F., Seijo L., Sen S., Sergentu D.-C., Stein C.J., Ungur L., Vacher M., Valentini A., Veryazov V., Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Centre National de la Recherche Scientifique (CNRS)-Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Centre National de la Recherche Scientifique (CNRS)-Institut de Chimie du CNRS (INC), École normale supérieure de Lyon (ENS de Lyon)-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Université de Lyon-Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS), Università degli studi della Tuscia [Viterbo], and Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Centre National de la Recherche Scientifique (CNRS)-Université Toulouse III - Paul Sabatier (UT3)
- Subjects
Code (set theory) ,Computer science ,molecular-dynamics ,Ab initio ,General Physics and Astronomy ,Physics, Atomic, Molecular & Chemical ,01 natural sciences ,analytical gradients ,Computational methods ,MATRIX RENORMALIZATION-GROUP ,Computer software ,Physics::Atomic Physics ,Wave function ,Excitation energies ,self-consistent-field ,010304 chemical physics ,Chemistry, Physical ,Physics ,Density matrix renormalization group ,AB-INITIO CALCULATIONS ,potential-energy surface ,[CHIM.THEO]Chemical Sciences/Theoretical and/or physical chemistry ,Chemistry ,ELECTRONIC-STRUCTURE ,Potential energy surfaces ,Physical Sciences ,Density functional theory ,Quantum chemistry ,Quantum mechanical/molecular mechanical calculations ,SELF-CONSISTENT-FIELD ,ab-initio calculations ,ANALYTICAL GRADIENTS ,electronic-structure ,transition-metal-complexes ,Electronic structure ,Quantum chemistry software, computational spectroscopy, computational photochemistry ,CASPT2 versus density functional theory ,Molecular dynamics ,010402 general chemistry ,reduced multiplication scheme ,Computational science ,0103 physical sciences ,Teoretisk kemi ,POTENTIAL-ENERGY SURFACE ,Physical and Theoretical Chemistry ,Theoretical Chemistry ,Science & Technology ,STATE PERTURBATION-THEORY ,matrix renormalization-group ,X-ray absorption spectroscopy ,TRANSITION-METAL-COMPLEXES ,state perturbation-theory ,0104 chemical sciences ,REDUCED MULTIPLICATION SCHEME ,MOLECULAR-DYNAMICS - Abstract
MOLCAS/OpenMolcas is an ab initio electronic structure program providing a large set of computational methods from Hartree-Fock and density functional theory to various implementations of multiconfigurational theory. This article provides a comprehensive overview of the main features of the code, specifically reviewing the use of the code in previously reported chemical applications as well as more recent applications including the calculation of magnetic properties from optimized density matrix renormalization group wave functions. ispartof: JOURNAL OF CHEMICAL PHYSICS vol:152 issue:21 ispartof: location:United States status: published
- Published
- 2020
7. Influence of the choice of projection manifolds in the CASPT2 implementation
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Takeshi Yanai, Jakub Chalupský, Per-Åke Malmqvist, Roland Lindh, Masaaki Saitow, and Yuki Kurashige
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Source code ,010304 chemical physics ,Degree (graph theory) ,Computer science ,media_common.quotation_subject ,Suite ,Biophysics ,Electronic structure ,010402 general chemistry ,Condensed Matter Physics ,01 natural sciences ,Projection (linear algebra) ,0104 chemical sciences ,0103 physical sciences ,Code (cryptography) ,Perturbation theory (quantum mechanics) ,Complete active space ,Physical and Theoretical Chemistry ,Molecular Biology ,Algorithm ,media_common - Abstract
The Complete Active Space Second-Order Perturbation Theory (CASPT2) is well-established as a high-accuracy electronic structure method. It was originally implemented in the early 1990s to an efficient computer code in the molcas program suite, and this implementation has been extensively used as a standard tool. Here, we report a comparison of it against two independent computer-aided implementations of the CASPT2 method, revealing that the CASPT2 energies provided by the original code of molcas (version 8 or earlier) are inconsistent with the predictions of the lately developed computer-aided implementations. It is shown that this error is associated with the projections of the first-order equation onto the fully internally contracted multireference bases which are partially inconsistent between the left- and right-hand sides. The degree of the errors is assessed by performing illustrative CASPT2 calculations. The errors in total CASPT2 energies are demonstrated to be negligible relative to chemical accuracy in many cases, while there is a difficult case where they may substantially alter chemical description. The incorporation of the consistent projections into molcas has been carried out, which is available in the version 8 sp1. (Less)
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- 2016
8. OpenMolcas: From Source Code to Insight
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Ignacio Fdez. Galván, Morgane Vacher, Ali Alavi, Celestino Angeli, Jochen Autschbach, Jie J. Bao, Sergey I. Bokarev, Nikolay A. Bogdanov, Rebecca K. Carlson, Liviu F. Chibotaru, Joel Creutzberg, Nike Dattani, Mickaël G. Delcey, Sijia Dong, Andreas Dreuw, Leon Freitag, Luis Manuel Frutos, Laura Gagliardi, Frédéric Gendron, Angelo Giussani, Leticia Gonzalez, Gilbert Grell, Meiyuan Guo, Chad E. Hoyer, Marcus Johansson, Sebastian Keller, Stefan knecht, Goran Kovačević, Erik Källman, Giovanni Li Manni, Marcus Lundberg, Yingjin Ma, Sebastian Mai, João Pedro Malhado, Per Åke Malmqvist, Philipp Marquetand, Stefanie A. Mewes, Jesper Norell, Massimo Olivucci, Markus Oppel, Quan Manh Phung, Kristine Pierloot, Felix Plasser, Markus Reiher, Andrew M. Sand, Igor Schapiro, Prachi Sharma, Christopher J. Stein, Lasse Kragh Sørensen, Donald G. Truhlar, Mihkel Ugandi, Liviu Ungur, Alessio Valentini, Steven Vancoillie, Valera Veryazov, Oskar Weser, Per-Olof Widmark, Sebastian Wouters, J. Patrick Zobel, and Roland Lindh
- Abstract
In this article we describe the OpenMolcas environment and invite the computational chemistry community to collaborate. The open-source project already includes a large number of new developments realized during the transition from the commercial MOLCAS product to the open-source platform. The paper initially describes the technical details of the new software development platform. This is followed by brief presentations of many new methods, implementations, and features of the OpenMolcas program suite. These developments include novel wave function methods such as stochastic complete active space self-consistent field, density matrix renormalization group (DMRG) methods, and hybrid multiconfigurational wave function and density functional theory models. Some of these implementations include an array of additional options and functionalities. The paper proceeds and describes developments related to explorations of potential energy surfaces. Here we present methods for the optimization of conical intersections, the simulation of adiabatic and nonadiabatic molecular dynamics and interfaces to tools for semiclassical and quantum mechanical nuclear dynamics. Furthermore, the article describes features unique to simulations of spectroscopic and magnetic phenomena such as the exact semiclassical description of the interaction between light and matter, various X-ray processes, magnetic circular dichroism and properties. Finally, the paper describes a number of built-in and add-on features to support the OpenMolcas platform with post calculation analysis and visualization, and new electronic and muonic basis sets.
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- 2019
9. OpenMolcas: From Source Code to Insight
- Author
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Per-Åke Malmqvist, Laura Gagliardi, Liviu F. Chibotaru, Nikolay A. Bogdanov, Rebecca K. Carlson, Valera Veryazov, Prachi Sharma, Sebastian Keller, Sebastian Wouters, Frédéric Gendron, Sebastian Mai, Alessio Valentini, Markus Reiher, Oskar Weser, Mihkel Ugandi, Stefanie A. Mewes, Erik Källman, Stefan Knecht, Sergey I. Bokarev, Liviu Ungur, Morgane Vacher, Angelo Giussani, Mickaël G. Delcey, Giovanni Li Manni, Kristine Pierloot, Joel Creutzberg, Nikesh S. Dattani, João Pedro Malhado, Goran Kovačević, Meiyuan Guo, Luis Manuel Frutos, Andrew M. Sand, J. Patrick Zobel, Alexander Zech, Tomasz Adam Wesolowski, Ignacio Fdez. Galván, Jie J. Bao, Massimo Olivucci, Jochen Autschbach, Marcus Johansson, Donald G. Truhlar, Leticia González, Per-Olof Widmark, Yingjin Ma, Igor Schapiro, Lasse Kragh Sørensen, Ali Alavi, Marcus Lundberg, Jesper Norell, Felix Plasser, Sijia S. Dong, Celestino Angeli, Christopher J. Stein, Quan Manh Phung, Gilbert Grell, Chad E. Hoyer, Markus Oppel, Leon Freitag, Francesco Aquilante, Philipp Marquetand, Andreas Dreuw, Steven Vancoillie, Roland Lindh, Angström Laboratory, Uppsala University, Department of Chemistry [Imperial College London], Imperial College London, Max Planck Institute for Solid State Research, Max-Planck-Gesellschaft, Dipartimento di Scienze Chimiche e Farmaceutiche, Università degli Studi di Ferrara (UniFE), Department of Chemistry [Buffalo], University at Buffalo [SUNY] (SUNY Buffalo), State University of New York (SUNY)-State University of New York (SUNY), China Agricultural University (CAU), Leibniz Institute for Solid State and Materials Research (IFW Dresden), Leibniz Association, Institute for Nanoscale Physics and Chemistry (INPAC), Université Catholique de Louvain = Catholic University of Louvain (UCL), Interdisciplinary Center for Scientific Computing (IWR), Universität Heidelberg [Heidelberg], University of Vienna [Vienna], Departamento de Química Física, Universidad de Alcalá - University of Alcalá (UAH), Department of Chemistry, Supercomputing Institute, and Chemical Theory Center, University of Minnesota [Twin Cities] (UMN), University of Minnesota System-University of Minnesota System, Instituto de Ciencia Molecular (ICMol), Universitat de València (UV), institut für Theoretische Chemie, Universität Wien, Universität Wien, Department of Physics, Chemistry and Pharmacy, University of Southern Denmark, Campusvej 55, 5230 Odense, Denmark, Processus d'Activation Sélective par Transfert d'Energie Uni-électronique ou Radiatif (UMR 8640) (PASTEUR), Université Pierre et Marie Curie - Paris 6 (UPMC)-Département de Chimie - ENS Paris, École normale supérieure - Paris (ENS Paris), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-École normale supérieure - Paris (ENS Paris), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS), Division of Theoretical Chemistry, Dipartimento di Chimica, Università degli Studi di Siena = University of Siena (UNISI), Loughborough University, Laboratorium für Physikalische Chemie (ETH-LPC), Eidgenössische Technische Hochschule - Swiss Federal Institute of Technology [Zürich] (ETH Zürich), Bowling Green State University (BGSU), Yerkes National Primate Research Center [Lawrenceville, GA], Emory University [Atlanta, GA], Division of Quantum and Chemistry, Catholic University of Leuven - Katholieke Universiteit Leuven (KU Leuven), Dipartimento di Produzioni Animali, Università della Tuscia, University of Silesia in Katowice, Department of Theoretical Chemistry, Lund University [Lund], and Department of Chemistry-Angstrom, the Theoretical Chemistry Programme
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Wave function ,Source code ,Field (physics) ,Computer science ,media_common.quotation_subject ,Interfaces ,Semiclassical physics ,010402 general chemistry ,0601 Biochemistry and Cell Biology ,01 natural sciences ,Computational science ,NO ,Chemical calculations ,Mathematical methods ,chemical calculations ,electron correlation ,interfaces ,mathematical methods ,wave function ,0103 physical sciences ,0307 Theoretical and Computational Chemistry ,Physical and Theoretical Chemistry ,Wave function, Interfaces, Chemical calculations, Mathematical methods, Electron correlation ,ComputingMilieux_MISCELLANEOUS ,media_common ,Chemical Physics ,010304 chemical physics ,Basis (linear algebra) ,business.industry ,Density matrix renormalization group ,Electron correlation ,Software development ,0803 Computer Software ,0104 chemical sciences ,Computer Science Applications ,Visualization ,[CHIM.THEO]Chemical Sciences/Theoretical and/or physical chemistry ,business - Abstract
In this article we describe the OpenMolcas environment and invite the computational chemistry community to collaborate. The open-source project already includes a large number of new developments realized during the transition from the commercial MOLCAS product to the open-source platform. The paper initially describes the technical details of the new software development platform. This is followed by brief presentations of many new methods, implementations, and features of the OpenMolcas program suite. These developments include novel wave function methods such as stochastic complete active space self-consistent field, density matrix renormalization group (DMRG) methods, and hybrid multiconfigurational wave function and density functional theory models. Some of these implementations include an array of additional options and functionalities. The paper proceeds and describes developments related to explorations of potential energy surfaces. Here we present methods for the optimization of conical intersections, the simulation of adiabatic and nonadiabatic molecular dynamics, and interfaces to tools for semiclassical and quantum mechanical nuclear dynamics. Furthermore, the article describes features unique to simulations of spectroscopic and magnetic phenomena such as the exact semiclassical description of the interaction between light and matter, various X-ray processes, magnetic circular dichroism, and properties. Finally, the paper describes a number of built-in and add-on features to support the OpenMolcas platform with postcalculation analysis and visualization, a multiscale simulation option using frozen-density embedding theory, and new electronic and muonic basis sets.
- Published
- 2019
10. Parallelization of a multiconfigurational perturbation theory
- Author
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Per-Åke Malmqvist, Valera Veryazov, Mickaël G. Delcey, Steven Vancoillie, Victor P. Vysotskiy, and Roland Lindh
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Interconnection ,Computer science ,InfiniBand ,Complex system ,Memory bandwidth ,General Chemistry ,Parallel computing ,Supercomputer ,Computational Mathematics ,Scalability ,Organometallic Compounds ,Bandwidth (computing) ,Quantum Theory ,Scaling - Abstract
In this work, we present a parallel approach to complete and restricted active space second-order perturbation theory, (CASPT2/RASPT2). We also make an assessment of the performance characteristics of its particular implementation in the Molcas quantum chemistry programming package. Parallel scaling is limited by memory and I/O bandwidth instead of available cores. Significant time savings for calculations on large and complex systems can be achieved by increasing the number of processes on a single machine, as long as memory bandwidth allows, or by using multiple nodes with a fast, low-latency interconnect. We found that parallel efficiency drops below 50% when using 8-16 cores on the shared-memory architecture, or 16-32 nodes on the distributed-memory architecture, depending on the calculation. This limits the scalability of the implementation to a moderate amount of processes. Nonetheless, calculations that took more than 3 days on a serial machine could be performed in less than 5 h on an InfiniBand cluster, where the individual nodes were not even capable of running the calculation because of memory and I/O requirements. This ensures the continuing study of larger molecular systems by means of CASPT2/RASPT2 through the use of the aggregated computational resources offered by distributed computing systems.
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- 2013
11. Multiconfigurational Quantum Chemistry
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Björn O. Roos, Roland Lindh, Per Åke Malmqvist, Valera Veryazov, and Per-Olof Widmark
- Published
- 2016
12. Potential Energy Surface of the Chromium Dimer Re-re-revisited with Multiconfigurational Perturbation Theory
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Valera Veryazov, Per-Åke Malmqvist, and Steven Vancoillie
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010304 chemical physics ,Chemistry ,Dimer ,Extrapolation ,010402 general chemistry ,01 natural sciences ,0104 chemical sciences ,Computer Science Applications ,symbols.namesake ,chemistry.chemical_compound ,Quantum mechanics ,0103 physical sciences ,Potential energy surface ,symbols ,Molecule ,Density functional theory ,Physics::Atomic Physics ,Physical and Theoretical Chemistry ,Wave function ,Hamiltonian (quantum mechanics) ,Basis set - Abstract
The chromium dimer has long been a benchmark molecule to evaluate the performance of different computational methods ranging from density functional theory to wave function methods. Among the latter, multiconfigurational perturbation theory was shown to be able to reproduce the potential energy surface of the chromium dimer accurately. However, for modest active space sizes, it was later shown that different definitions of the zeroth-order Hamiltonian have a large impact on the results. In this work, we revisit the system for the third time with multiconfigurational perturbation theory, now in order to increase the active space of the reference wave function. This reduces the impact of the choice of zeroth-order Hamiltonian and improves the shape of the potential energy surface significantly. We conclude by comparing our results of the dissocation energy and vibrational spectrum to those obtained from several highly accurate multiconfigurational methods and experiment. For a meaningful comparison, we used the extrapolation to the complete basis set for all methods involved.
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- 2016
13. Molcas 8: New capabilities for multiconfigurational quantum chemical calculations across the periodic table
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Liviu Ungur, Felix Plasser, Artur Nenov, Michael Stenrup, Mickaël G. Delcey, Laura Gagliardi, Giovanni Li Manni, Thomas Bondo Pedersen, Luis Manuel Frutos, Thomas Müller, Felipe Zapata, Donald G. Truhlar, Jochen Autschbach, Luca De Vico, Valera Veryazov, Francesco Aquilante, Liviu F. Chibotaru, Ivan Rivalta, Chad E. Hoyer, Dongxia Ma, Marco Garavelli, Massimo Olivucci, Ben Pritchard, Ignacio Fdez. Galván, Roland Lindh, Victor P. Vysotskiy, Oliver Weingart, Alessio Valentini, Per-Åke Malmqvist, Rebecca K. Carlson, Angelo Giussani, Nicolas Ferré, Igor Schapiro, Javier Segarra-Martí, Markus Reiher, Daoling Peng, Steven Vancoillie, Hans Lischka, Department of Chemistry [Buffalo], University at Buffalo [SUNY] (SUNY Buffalo), State University of New York (SUNY)-State University of New York (SUNY), Universidade Federal de Santa Catarina = Federal University of Santa Catarina [Florianópolis] (UFSC), Institute for Nanoscale Physics and Chemistry (INPAC), Université Catholique de Louvain = Catholic University of Louvain (UCL), Department of Chemistry-Angstrom, the Theoretical Chemistry Programme, Uppsala University, Institut de Chimie Radicalaire (ICR), Aix Marseille Université (AMU)-Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS), Department of Chemistry, Supercomputing Institute, and Chemical Theory Center, University of Minnesota [Twin Cities] (UMN), University of Minnesota System-University of Minnesota System, Production,Landscape,Agroenergy, Università degli Studi di Milano = University of Milan (UNIMI), China Agricultural University (CAU), Institute for theoretical Chemistry, University of Vienna [Vienna], Division of Theoretical Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences [Tel Aviv] (TAU), Tel Aviv University (TAU), Alma Mater Studiorum Università di Bologna [Bologna] (UNIBO), Dipartimento di Chimica, Università degli Studi di Siena = University of Siena (UNISI), Aalborg University [Denmark] (AAU), Laboratorium für Physikalische Chemie (ETH-LPC), Eidgenössische Technische Hochschule - Swiss Federal Institute of Technology [Zürich] (ETH Zürich), Laboratoire de Chimie - UMR5182 (LC), École normale supérieure de Lyon (ENS de Lyon)-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Université de Lyon-Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS), University of Minnesota System, Division of Quantum and Chemistry, Catholic University of Leuven - Katholieke Universiteit Leuven (KU Leuven), Vitrociset S.p.a., University of Milan, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University [Tel Aviv], Dipartimento di Chimica, 'G.Ciamician' Università di Bologna, Centre National de la Recherche Scientifique (CNRS)-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Université de Lyon-École normale supérieure - Lyon (ENS Lyon)-Institut de Chimie du CNRS (INC), Aquilante, Francesco, Autschbach, Jochen, Carlson, Rebecca K., Chibotaru, Liviu F., Delcey, Mickaël G., De Vico, Luca, Ignacio, Fdez. Galván, Ferré, Nicola, Frutos, Luis Manuel, Gagliardi, Laura, Garavelli, Marco, Giussani, Angelo, Hoyer, Chad E., Li Manni, Giovanni, Lischka, Han, Ma, Dongxia, Malmqvist, Per Åke, Müller, Thoma, Nenov, Artur, Olivucci, Massimo, Pedersen, Thomas Bondo, Peng, Daoling, Plasser, Felix, Pritchard, Ben, Reiher, Marku, Rivalta, Ivan, Schapiro, Igor, Segarra-Martí, Javier, Stenrup, Michael, Truhlar, Donald G., Ungur, Liviu, Valentini, Alessio, Vancoillie, Steven, Veryazov, Valera, Vysotskiy, Victor P., Weingart, Oliver, Zapata, Felipe, and Lindh, Roland
- Subjects
gradients ,Macrocyclic Compounds ,Computation ,Surface hopping ,Electrons ,Molecular Dynamics Simulation ,electron correlation ,010402 general chemistry ,01 natural sciences ,Atomic orbital ,parallelization ,0103 physical sciences ,Teoretisk kemi ,Linear scale ,Statistical physics ,Complete active space ,Wave function ,Theoretical Chemistry ,Physics ,010304 chemical physics ,molecular dynamic ,Chemistry (all) ,Multireference configuration interaction ,General Chemistry ,molecular dynamics ,0104 chemical sciences ,Computational physics ,gradient ,[CHIM.THEO]Chemical Sciences/Theoretical and/or physical chemistry ,Computational Mathematics ,relativistic ,Quantum Theory ,Thermodynamics ,Density functional theory ,Algorithms ,Software ,Thymidine - Abstract
In this report, we summarize and describe the recent unique updates and additions to the Molcas quantum chemistry program suite as contained in release version 8. These updates include natural and spin orbitals for studies of magnetic properties, local and linear scaling methods for the Douglas-Kroll-Hess transformation, the generalized active space concept in MCSCF methods, a combination of multiconfigurational wave functions with density functional theory in the MC-PDFT method, additional methods for computation of magnetic properties, methods for diabatization, analytical gradients of state average complete active space SCF in association with density fitting, methods for constrained fragment optimization, large-scale parallel multireference configuration interaction including analytic gradients via the interface to the Columbus package, and approximations of the CASPT2 method to be used for computations of large systems. In addition, the report includes the description of a computational machinery for nonlinear optical spectroscopy through an interface to the QM/MM package Cobramm. Further, a module to run molecular dynamics simulations is added, two surface hopping algorithms are included to enable nonadiabatic calculations, and the DQ method for diabatization is added. Finally, we report on the subject of improvements with respects to alternative file options and parallelization. QC 20210506
- Published
- 2016
14. The binatural orbitals of electronic transitions
- Author
-
Per-Åke Malmqvist and Valera Veryazov
- Subjects
Density matrix ,Chemistry ,Biophysics ,Molecular orbital theory ,Condensed Matter Physics ,Slater-type orbital ,Linear combination of atomic orbitals ,Quantum electrodynamics ,Quantum mechanics ,Molecular orbital ,Complete active space ,Physical and Theoretical Chemistry ,Molecular Biology ,Basis set ,Natural bond orbital - Abstract
The well-known natural orbitals are defined as eigenfunctions of a one-particle reduced density operator, and can be obtained from a computed density matrix by diagonalization. Similarly, in this article we define the binatural orbitals, which are obtained for a pair of wave functions by a singular value decomposition of a reduced transition density matrix. The pair of states would usually be eigenstates of the electronic Hamiltonian, and the binatural orbitals then serve as a useful tool for the analysis of the transition between these states. More generally, application to any two state functions gives important information as to how the two states differ. Some examples are shown.
- Published
- 2012
15. Cholesky Decomposition-Based Multiconfiguration Second-Order Perturbation Theory (CD-CASPT2): Application to the Spin-State Energetics of Co(III)(diiminato)(NPh)
- Author
-
Björn Olof Roos, Abhik Ghosh, Thomas Bondo Pedersen, Per-Åke Malmqvist, and Francesco Aquilante
- Subjects
Physics ,Computational chemistry ,Ab initio quantum chemistry methods ,Ab initio ,Density functional theory ,Electronic structure ,Physical and Theoretical Chemistry ,Perturbation theory ,Wave function ,Molecular physics ,Basis set ,Computer Science Applications ,Cholesky decomposition - Abstract
The electronic structure and low-lying electronic states of a Co(III)(diiminato)(NPh) complex have been studied using multiconfigurational wave function theory (CASSCF/CASPT2). The results have been compared to those obtained with density functional theory. The best agreement with ab initio results is obtained with a modified B3LYP functional containing a reduced amount (15%) of Hartree-Fock exchange. A relativistic basis set with 869 functions has been employed in the most extensive ab initio calculations, where a Cholesky decomposition technique was used to overcome problems arising from the large size of the two-electron integral matrix. It is shown that this approximation reproduces results obtained with the full integral set to a high accuracy, thus opening the possibility to use this approach to perform multiconfigurational wave-function-based quantum chemistry on much larger systems relative to what has been possible until now.
- Published
- 2015
16. How to select active space for multiconfigurational quantum chemistry?
- Author
-
Valera Veryazov, Per-Åke Malmqvist, and Björn O. Roos
- Subjects
Field (physics) ,Chemistry ,Condensed Matter Physics ,Quantum chemistry ,Atomic and Molecular Physics, and Optics ,Set (abstract data type) ,Algebra ,Development (topology) ,Atomic orbital ,Quantum mechanics ,Complete active space ,Physical and Theoretical Chemistry ,Perturbation theory ,Quantum - Abstract
Bjorn Roos is one of the pioneers in the development and usage of multiconfigurational methods, in particular, the complete active space self-consistent field method and the perturbational complete active space perturbation theory through second order. To perform multiconfigurational calculations using these methods, a set of active orbitals must be selected, and the success of the methods depends on the choice of this set. This is not only sometimes easy but also sometimes difficult, especially for use of the more recent RASSCF and RASPT2 methods (which use a "restricted active space" rather than the complete one). Although an automated procedure for selecting the active orbitals would be a preferable solution, this does not seem feasible yet. An account of the problem is given, with examples and some approaches that usually work. (C) 2011 Wiley Periodicals, Inc. Int J Quantum Chem 111: 3329-3338, 2011 (Less)
- Published
- 2011
17. Infrared Spectra and Quantum Chemical Calculations of the Uranium Carbide Molecules UC and CUC with Triple Bonds
- Author
-
Benjamin Villeroy, Per-Åke Malmqvist, António Pereira Gonçalves, C. Godart, Björn O. Roos, Joaquim Marçalo, Cláudia C. L. Pereira, Xuefeng Wang, and Lester Andrews
- Subjects
Chemistry ,General Chemistry ,Triple bond ,Biochemistry ,Bond order ,Diatomic molecule ,Catalysis ,Bond length ,Colloid and Surface Chemistry ,Molecule ,Single bond ,Molecular orbital ,Atomic physics ,Ground state - Abstract
Laser evaporation of carbon-rich uranium/carbon alloys followed by atom reactions in a solid argon matrix and trapping at 8 K gives weak infrared absorptions for CUO at 852 and 804 cm(-1). A new band at 827 cm(-1) becomes a doublet with mixed carbon 12 and 13 isotopes and exhibits the 1.0381 isotopic frequency ratio, which is appropriate for the UC diatomic molecule, and another new band at 891 cm(-1) gives a three-band mixed isotopic spectrum with the 1.0366 isotopic frequency ratio, which is characteristic of the linear CUC molecule. CASPT2 calculations with dynamical correlation find the C[triple bond]U[triple bond]C ground state as linear 3Sigma(u)+ with 1.840 A bond length and molecular orbital occupancies for an effective bond order of 2.83. Similar calculations with spin-orbit coupling show that the U[triple bond]C diatomic molecule has a quintet (Lambda = 5, Omega = 3) ground state, a similar 1.855 A bond length, and a fully developed triple bond of 2.82 effective bond order.
- Published
- 2010
18. MOLCAS 7: The Next Generation
- Author
-
Michal Pitonak, Valera Veryazov, Björn O. Roos, Francesco Aquilante, Roland Lindh, Pavel Neogrády, Luca De Vico, Markus Reiher, Luis Serrano-Andrés, Per-Åke Malmqvist, Miroslav Urban, Thomas Bondo Pedersen, Giovanni Ghigo, and Nicolas Ferré
- Subjects
010304 chemical physics ,Field (physics) ,Basis (linear algebra) ,Photochemistry ,Computer science ,Context (language use) ,General Chemistry ,010402 general chemistry ,01 natural sciences ,0104 chemical sciences ,Computational Mathematics ,Coupled cluster ,Atomic orbital ,Computational chemistry ,0103 physical sciences ,Quantum Theory ,Statistical physics ,Perturbation theory (quantum mechanics) ,Wave function ,Algorithms ,Software ,Cholesky decomposition - Abstract
Some of the new unique features of the MOLCAS quantum chemistry package version 7 are presented inthis report. In particular, the Cholesky decomposition method applied to some quantum chemical methods is described. This approach is used both in the context of a straight forward approximation of the two-electron integrals and in the generation of so-called auxiliary basis sets. The article describes how the method is implemented for most known wave functions models: self-consistent field, density functional theory, 2nd order perturbation theory, complete-active space self-consistent field multiconfigurational reference 2nd order perturbation theory, and coupled-cluster methods. The report further elaborates on the implementation of a restricted-active space self-consistent field reference function in conjunction with 2nd order perturbation theory. The average atomic natural orbital basis for relativistic calculations, covering the whole periodic table, are described and associated unique properties are demonstrated. Furthermore, the use of the arbitrary order Douglas-Kroll-Hess transformation for one-component relativistic calculations and its implementation are discussed. This section especially focuses on the implementation of the so-called picture-change-free atomic orbital property integrals. Moreover, the ElectroStatic Potential Fitted scheme, a version of a quantum mechanics/molecular mechanics hybrid method implemented in MOLCAS, is described and discussed. Finally, the report discusses the use of the MOLCAS package for advanced studies of photo chemical phenomena and the usefulness of the algorithms for constrained geometry optimization in MOLCAS in association with such studies. (c) 2009 Wiley Periodicals, Inc. J Corn put Chem 31: 224-247, 2010
- Published
- 2010
19. Experimental and Theoretical Investigation of Simple Terminal Group 6 Arsenide As≡MF3 Molecules
- Author
-
Xuefeng Wang, Per-Åke Malmqvist, Marta Knitter, Lester Andrews, and Björn O. Roos
- Subjects
chemistry.chemical_compound ,Crystallography ,Valence (chemistry) ,chemistry ,Atomic orbital ,Atom ,Molecule ,Density functional theory ,Physical and Theoretical Chemistry ,Atomic physics ,Triple bond ,Bond order ,Arsenide - Abstract
Laser-ablated group 6 metal atoms react with NF3 and PF3 to form the simple lowest energy N MF3 and P=MX3 products, and this investigation has been extended to AsF3. Mo and W atoms react with AsF3 upon excitation by laser ablation or UV irradiation to form stable trigonal As MF3 terminal arsenides. These molecules are identified by comparison of the closely related infrared spectra of the analogous phosphide species and with frequencies calculated by density functional theory and multiconfigurational second order perturbation theory (CASSCF/CASPT2). Computed CASSCF/CASPT2 triple bond lengths for the As MoF3 and As WF3 molecules are 2.240 angstrom and 2.250 angstrom, respectively. The natural bond orders calculated by CASSCF/CASPT2 decrease from 2.67 to 2.60 for P MoF3 to As MoF3 and from 2.74 to 2.70 for P WF3 to As WF3 as the arsenic valence orbitals are less effective than those of phosphorus in bonding to each metal atom and the larger metal orbital size becomes more compatible with the arsenic valence orbitals. The Cr atom reaction gives the arsinidene AsF=CrF2 product instead of the higher energy As CrF3 molecule as the Cr (VI) state is not supported by the softer pnictides. (Less)
- Published
- 2009
20. Role of electronic curve crossing of benzeneS1state in the photodissociation of aryl halides, effect of fluorination: RASSI-SO MS-CASPT2 study
- Author
-
Ajitha Devarajan, Alexander Gaenko, Roland Lindh, and Per-Åke Malmqvist
- Subjects
Iodobenzene ,Photodissociation ,Ab initio ,Quantum yield ,Condensed Matter Physics ,Potential energy ,Atomic and Molecular Physics, and Optics ,chemistry.chemical_compound ,chemistry ,Physics::Atomic and Molecular Clusters ,Complete active space ,Singlet state ,Physics::Chemical Physics ,Physical and Theoretical Chemistry ,Atomic physics ,Benzene - Abstract
An ab initio study of the role of electronic curve crossing of benzene S-1 state in the photo dissociation dynamics of the iodobenzene and effect of fluorination is presented. Two dissociative life times observed in iodobenzene is attributed to the coupled repulsive potential energy curves of the low-lying n-sigma*, pi-sigma*, pi-pi* states. The direct channel is attributed to the alkyl like transition and the indirect channel is attributed to the mixing of the alkyl like transitions with the low lying benzene pi-pi* transitions. Fluorination of iodobenzene results in a substantial increase in the direct channel product. To analyze the possible role of electronic curve crossing of these transitions, potential energy curves of low-lying n-sigma*, pi-sigma*, pi-pi* states were studied including spin-orbit and relativistic effects using the Restricted Active Space state interaction multistate complete active space perturbation theory (RASSI-MS-CASPT2) method. Crossing behavior of spin-free and spin-orbit potential energy curves was analyzed for the role of the benzene S-1 state. Our results indicate the curve crossing region to be around 2.00-2.35 angstrom for both C6H5I and C6F5I. Analysis of effect of fluorination on the energies of states corresponding to benzene pi-pi* and n-sigma* transitions suggests an increase in the energy of benzene pi-pi* states and a decrease in the energy of the states corresponding to n-sigma* transitions. Increased spin-orbit gap, increased separation of the benzene S-1(pi-pi*) state and n-sigma* states in the region of curve crossing, lesser mixing of the pi-pi* and n-sigma* states, an order of magnitude decrease in the transition strength to the benzene singlet transition all contributed to the observed Substantial increase in the quantum yield of the direct channel product on fluorination of aryl halides. (c) 2009 Wiley Periodicals, Inc. Int J Quantum Chem 109: 1962-1974, 2009 (Less)
- Published
- 2009
21. Calculation of EPR g Tensors for Transition-Metal Complexes Based on Multiconfigurational Perturbation Theory (CASPT2)
- Author
-
Per-Åke Malmqvist, Steven Vancoillie, and Kristine Pierloot
- Subjects
Coupling constant ,Zeeman effect ,Chemistry ,Triatomic molecule ,Electronic structure ,Atomic and Molecular Physics, and Optics ,symbols.namesake ,Computational chemistry ,Ab initio quantum chemistry methods ,Theoretical chemistry ,symbols ,Physical chemistry ,Density functional theory ,Physical and Theoretical Chemistry ,Perturbation theory - Abstract
The computation of the electronic g tensor by two multireference methods is presented and applied to a selection of molecules including CN, BO, AlO, GaO, InO, ZnH, ZnF, O(2), H(2)O(+), O(3) (-), and H(2)CO(+) (group A) as well as TiF(3), CuCl(4) (2-), Cu(NH(3))(4) (2+), and a series of d(1)-MOX(4) (n-) compounds, with M=V, Cr, Mo, Tc, W, Re and X=F, Cl, Br (group B). Two approaches are considered, namely, one in which spin-orbit coupling and the Zeeman effect are included using second-order perturbation theory and another one in which the Zeeman effect is added through first-order degenerate perturbation theory within the ground-state Kramers doublet. The two methods have been implemented into the MOLCAS quantum chemistry software package. The results obtained for the molecules in group A are in good agreement with experiment and with previously reported calculated g values. The results for the molecules in group B vary. While the g values for the d(1) systems are superior to previous theoretical results, those obtained for the d(9) systems are too large compared to the experimental values.
- Published
- 2007
22. Infrared Spectrum and Bonding in Uranium Methylidene Dihydride, CH2UH2
- Author
-
Jonathan T. Lyon, Lester Andrews, Bruce E. Bursten, Björn O. Roos, Per-Åke Malmqvist, and Tianxiao Yang
- Subjects
Uranium hydride ,Agostic interaction ,Hydrogen ,Chemistry ,Infrared ,chemistry.chemical_element ,Thorium ,Uranium ,Photochemistry ,Inorganic Chemistry ,chemistry.chemical_compound ,Molecule ,Physical chemistry ,Density functional theory ,Physical and Theoretical Chemistry - Abstract
Uranium atoms activate methane upon ultraviolet excitation to form the methyl uranium hydride CH3-UH, which undergoes alpha-H transfer to produce uranium methylidene dihydride, CH2UH2. This rearrangement most likely occurs on an excited-quintet potential-energy surface and is followed by relaxation in the argon matrix. These simple U + CH4 reaction products are identified through isotopic substitution ((CH4)-C-13, CD4, CH2D2) and density functional theory frequency and structure calculations for the strong U-H stretching modes. Relativistic multiconfiguration (CASSCF/CASPT2) calculations substantiate the agostic distorted C-1 ground-state structure for the triplet CH2UH2 molecule. We find that uranium atoms are less reactive in methane activation than thorium atoms. Our calculations show that the CH2UH2 complex is distorted more than CH2ThH2. A favorable interaction between the low energy open-shell U(5f) sigma orbital and the agostic hydrogen contributes to the distortion in the uranium methylidene complexes. (Less)
- Published
- 2007
23. New relativistic ANO basis sets for actinide atoms
- Author
-
Valera Veryazov, Roland Lindh, Per-Olof Widmark, Björn O. Roos, and Per-Åke Malmqvist
- Subjects
Chemistry ,General Physics and Astronomy ,Ion ,Polarizability ,Excited state ,Atom ,Physics::Atomic and Molecular Clusters ,Physics::Atomic Physics ,Physical and Theoretical Chemistry ,Atomic physics ,Ionization energy ,Relativistic quantum chemistry ,Wave function ,Excitation - Abstract
New basis sets of the atomic natural orbital (ANO) type have been developed for the actinide atoms Ac-Cm. The ANOs have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive ions, and the atom in a electric field. Scalar relativistic effects are included through the use of a Douglas-Kroll-Hess Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies and some excitation energies. Computed ionization energies have an accuracy better than 0.2 eV in most cases. The lowest multiplet levels have been computed. These calculations include spin-orbit coupling using a variation-perturbation approach. The atomic polarizability of the spherically symmetric americium atom has been computed to be 116 au(3). (c) 2005 Elsevier B.V. All rights reserved.
- Published
- 2005
24. Using on-top pair density for construction of correlation functionals for multideterminant wave functions
- Author
-
Roland Lindh, Per-Åke Malmqvist, and Sergey Gusarov
- Subjects
Chemistry ,Orbital-free density functional theory ,Biophysics ,Probability density function ,Condensed Matter Physics ,Diatomic molecule ,Fock matrix ,Quantum mechanics ,Physics::Atomic and Molecular Clusters ,Density functional theory ,Complete active space ,Physics::Chemical Physics ,Physical and Theoretical Chemistry ,Atomic physics ,Wave function ,Molecular Biology ,Excitation - Abstract
The value of the two-particle density function at coalescence is frequently used as an additional variable for formulating approximate exchange-correlation or correlation functionals. Here, its applications as one of the key variables for the construction of new DFT (preferably multi-determinant) functionals is investigated. The basic formalism is presented and it is shown that this replacement avoids some difficulty to construct a Fock matrix in a ROKS (restricted open-shell Kohn–Sham) method and also to reduce the ‘double counting’ of correlation energy in CASDFT (complete active space density functional theory) calculations. Calculations of excitation energies for transition metals and dissociation curves for diatomic molecules are presented as an example.
- Published
- 2004
25. Spin–orbit ab initio study of alkyl halide dissociation via electronic curve crossing
- Author
-
Małgorzata Wierzbowska, D Ajitha, Per-Åke Malmqvist, and Roland Lindh
- Subjects
Intersystem crossing ,Ab initio quantum chemistry methods ,Chemistry ,Excited state ,Photodissociation ,Ab initio ,General Physics and Astronomy ,Physics::Chemical Physics ,Physical and Theoretical Chemistry ,Atomic physics ,Potential energy ,Dissociation (chemistry) ,Basis set - Abstract
An ab initio study of the role of electronic curve crossing in the photodissociation dynamics of the alkyl halides is presented. Recent experimental studies show that curve crossing plays a deterministic role in deciding the channel of dissociation. Coupled repulsive potential energy curves of the low-lying n-sigma(*) states are studied including spin-orbit and relativistic effects. Basis set including effect of core correlation is used. Ab initio vertical excitation spectra of CH(3)I and CF(3)I are in agreement with the experimental observation. The curve crossing region is around 2.371 A for CH(3)I and CF(3)I. The potential curves of the repulsive excited states have larger slope for CF(3)I, suggesting a higher velocity and decreased intersystem crossing probability on fluorination. We also report the potential curves and the region of curve crossing for CH(3)Br and CH(3)Cl.
- Published
- 2004
26. A modified definition of the zeroth-order Hamiltonian in multiconfigurational perturbation theory (CASPT2)
- Author
-
Björn O. Roos, Giovanni Ghigo, and Per-Åke Malmqvist
- Subjects
Chemistry ,General Physics and Astronomy ,Diatomic molecule ,Dissociation (chemistry) ,Zeroth order ,symbols.namesake ,Excited state ,Quantum mechanics ,symbols ,Molecule ,Physics::Chemical Physics ,Physical and Theoretical Chemistry ,Hamiltonian (quantum mechanics) ,Open shell ,Excitation - Abstract
A new shifted zeroth-order Hamiltonian is presented, which will be used in second-order multiconfigurational perturbation theory (CASPT2). The new approximation corrects for the systematic error of the original formulation, which led to an relative overestimate of the correlation energy for open shell system, resulting in too small dissociation and excitation energies. Errors in the D, values for 49 diatomic molecules have been reduced with more than 50%. Calculations on excited states of the N-2 and benzene molecules give a similar improvement. (C) 2004 Elsevier B.V. All rights reserved.
- Published
- 2004
27. Correlation potentials for a multiconfigurational-based density functional theory with exact exchange
- Author
-
Sergey Gusarov, Björn O. Roos, Roland Lindh, and Per-Åke Malmqvist
- Subjects
Correlation function (statistical mechanics) ,Orbital-free density functional theory ,Ab initio quantum chemistry methods ,Chemistry ,Quantum mechanics ,Density functional theory ,Probability density function ,Complete active space ,Physical and Theoretical Chemistry ,Electronic density ,Hybrid functional - Abstract
A density functional theory based on a complete active space self consistent field (CASSCF) reference function with exact exchange is discussed. It is first shown that such a theory may be formulated with a correlation potential dependent on the density function and on the active space used. Auxiliary functions, such as the on-top two-particle density, are used to define uniquely the potential for different active spaces. The paper also analyses the correlation functional for some atomic and molecular cases. Large ab initio calculations are performed to obtain accurate density functions. A correlation potential is then fitted such that the reference CASSCF function gives the same density. The correlation potential values are saved in a data base for future analysis.
- Published
- 2004
28. Main Group Atoms and Dimers Studied with a New Relativistic ANO Basis Set
- Author
-
Per-Olof Widmark, Valera Veryazov, Björn O. Roos, Roland Lindh, and Per-Åke Malmqvist
- Subjects
Density matrix ,Chemistry ,Ion ,symbols.namesake ,Excited state ,Physics::Atomic and Molecular Clusters ,symbols ,Physics::Atomic Physics ,Physical and Theoretical Chemistry ,Ionization energy ,Atomic physics ,Hamiltonian (quantum mechanics) ,Relativistic quantum chemistry ,Wave function ,Basis set - Abstract
New basis sets of the atomic natural orbital (ANO) type have been developed for the main group and rare gas atoms. The ANO's have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive and negative ions, and the dimer at its equilibrium geometry. Scalar relativistic effects are included through the use of a Douglas−Kroll Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second-order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies, electron affinities, and excitation energies for all atoms and the ground-state potentials for the dimers. These calculations include spin−orbit coupling using the RASSCF State Interaction (RASSI-SO) method. The spin−orbit splitting for the lowest atomic term is reproduced with an accuracy of better than 0.05 eV, except for row 5, where it is 0.15 eV. Ionization energies and electron affinities have an accuracy bett...
- Published
- 2003
29. MOLCAS: a program package for computational chemistry
- Author
-
Per-Åke Malmqvist, Pavel Neogrády, Per-Olof Widmark, Björn O. Roos, Bernd Schimmelpfennig, Valera Veryazov, Maurizio Cossi, Ulf Ryde, Luis Seijo, Gunnar Karlström, and Roland Lindh
- Subjects
General Computer Science ,Chemistry ,General Physics and Astronomy ,General Chemistry ,Electronic structure ,Energy minimization ,Quantum chemistry ,Computational Mathematics ,Molecular dynamics ,Mechanics of Materials ,Computational chemistry ,Excited state ,Molecule ,General Materials Science ,Relativistic quantum chemistry ,Wave function - Abstract
The program system MOLCAS is a package for calculations of electronic and structural properties of molecular systems in gas, liquid, or solid phase. It contains a number of modern quantum chemical methods for studies of the electronic structure in ground and excited electronic states. A macromolecular environment can be modeled by a combination of quantum chemistry and molecular mechanics. It is further possible to describe a crystalline material using model potentials. Solvent effects can be treated using continuum models or by combining quantum chemical calculations with molecular dynamics or Monte-Carlo simulations. MOLCAS is especially adapted to treat systems with a complex electronic structure, where the simplest quantum chemical models do not work. These features together with the inclusion of relativistic effects makes it possible to treat with good accuracy systems including atoms from the entire periodic system. MOLCAS has effective methods for geometry optimization of equilibria, transition states, conical intersections, etc. This facilitates studies of excited state energy surfaces, spectroscopy, and photochemical processes.
- Published
- 2003
30. Theoretical Study of the Lowest 1BU States of trans-Stilbene
- Author
-
Laura Gagliardi, Giorgio Orlandi, Per-Åke Malmqvist, Björn O. Roos, and Vicent Molina
- Subjects
Absorption spectroscopy ,Chemistry ,Band gap ,Excited state ,ddc:540 ,Electron ,Physical and Theoretical Chemistry ,Atomic physics ,Ground state ,Adiabatic process ,HOMO/LUMO ,Excitation - Abstract
The results of a theoretical study of the ground state, 1(1)A(g), and of the lowest B-1(u) states of trans-stilbene are presented. The vertical and adiabatic excitation energies of the lowest B-1(u) states have been computed using multiconfigurational SCF theory, followed by second-order perturbation. theory. It is shown that the two lowest excited states are separated by a small energy gap in the Franck-Condon region. They are the 1(1)B(u), characterized by the HOMO-->LUMO single excitation substantially localized on the ethylenic moiety, and the 2(1)B(u), formed by a combination,of one electron excitations localized mainly on the benzene rings. The most intense transition is found to be the lowest in energy when the interaction between different states is included at the level of second-order perturbation theory. The vibronic structure of emission and absorption spectra of the two lowest B-1(u) states have been determined within the Franck-Condon approximation. The spectrum calculated for the 1(1)B(u) state agrees with the experimental spectrum, while the low intensity band computed for the 2(1)B(u) state has no experimental counterpart. It is concluded that this band is buried in the strong 1(1)B(u) absorption and therefore not observed. (Less)
- Published
- 2002
31. A linear response approach to second-order electronic transition intensities for multiconfigurational self-consistent field wave functions
- Author
-
Anders Bernhardsson, Jonna Stålring, and Per-Åke Malmqvist
- Subjects
Coupling ,Matrix (mathematics) ,Photon ,Field (physics) ,Chemistry ,Atomic electron transition ,Excited state ,Quantum mechanics ,General Physics and Astronomy ,Complete active space ,Physical and Theoretical Chemistry ,Atomic physics ,Wave function - Abstract
A new theoretical approach to two-photon transition intensities at the multiconfigurational self-consistent field (MCSCF) level of theory, is described in detail. The fundamental property of an MCSCF wave function, that it is possible to define the response equations for an excited state, is a prerequisite. The method requires solely first-order multiconfigurational response calculations, because the equations involve the response of both the initial and final state. However, the method is approximate as the coupling between the +ω and −ω parts of the linear response is disregarded. The complete active space state interaction (CASSI) method is applied in the evaluation of the involved matrix elements. To illustrate the performance and the requirements of this method, it was used to determine TP transitions in trans-1,3-butadiene and trans-stilbene.
- Published
- 2002
32. A theoretical study of the 21Ag← 11Agtwo-photon transition and its vibronic band intrans-stilbene
- Author
-
Per-Åke Malmqvist, Roland Lindh, Jonna Stålring, and Laura Gagliardi
- Subjects
Chemical Physics ,Thermodynamics &Kinetic Theory ,Field (physics) ,Chemistry ,Biophysics ,Condensed Matter Physics ,Physical Chemistry ,Theoretical Physics ,ddc:540 ,Group Theory ,Atomic &Nuclear Physics ,Complete active space ,Physical and Theoretical Chemistry ,Atomic physics ,Molecular Biology ,Mathematical Physics ,Quantum Mechanics - Abstract
The two-photon spectrum of the 2 1 A g ı1 1 A g transition in trans -stilbene has been calculated at the complete active space self-consistent field (CASSCF) level of theory. Energies were obtained at the complete active space second-order perturbation (CASPT2) level of theory, while the geometries of both the initial and final states were optimized at the CASSCF level. The energy and the geometry optimizations were performed using an active space of 14 electrons in 14 active πorbitals. The vibrational frequencies of both states and the two-photon transition (TPT) cross-section were calculated with a smaller active space where the two lowest πorbitals were kept inactive. A newly implemented algorithm, in the quantum chemical package Molcas was used to determine the two-photon transition intensity. This method requires only the linear response of the CASSCF wavefunction. Furthermore, the vibronic structure of this TPT was studied. The Franck-Condon factors were obtained by calculating the overlap between the vibrational states involved, which were determined from the force fields of both the initial and final states, at the CASSCF level of theory. The results are in agreement with experiment.
- Published
- 2002
33. The restricted active space (RAS) state interaction approach with spin–orbit coupling
- Author
-
Per-Åke Malmqvist, Björn O. Roos, and Bernd Schimmelpfennig
- Subjects
Hamiltonian matrix ,Chemistry ,Atoms in molecules ,General Physics and Astronomy ,Spin–orbit interaction ,Electronic structure ,Diatomic molecule ,symbols.namesake ,Mean field theory ,Quantum mechanics ,symbols ,Physical and Theoretical Chemistry ,Atomic physics ,Relativistic quantum chemistry ,Hamiltonian (quantum mechanics) - Abstract
A method to compute spin-orbit coupling between electronic states is presented. An effective one-electron spin-orbit Hamiltonian is used, based on atomic mean field integrals, The basic electronic states are obtained using the restricted active space (RAS) SCF method. The Hamiltonian matrix is obtained by an extension of the restricted active space state interaction (RASSI) method. Several hundred states can be included. Tests for atoms and molecules from the entire periodic system show accurate results. Computed spin-orbit effects on relative energies are normally accurate within a few percent. The method has been included in the MOLCAS-5.0 quantum chemistry software. (C) 2002 Elsevier Science B.V. All rights reserved.
- Published
- 2002
34. On the Electronic Structure of the UO2 Molecule
- Author
-
John M. Dyke, Per Åke Malmqvist, Laura Gagliardi, and Björn O. Roos
- Subjects
Bond length ,Chemistry ,Antisymmetric relation ,ddc:540 ,Molecule ,Electronic structure ,Physical and Theoretical Chemistry ,Atomic physics ,Ionization energy ,Adiabatic process ,Wave function ,Ground state - Abstract
The structure and vibrational frequencies of the UO[2] molecule have been determined using multiconfigurational wave functions (CASSCF/CASPT2), together with a newly developed method to treat spin−orbit coupling. The molecule has been found to have a (5f[phi])(7s), [3]Φ[u], Ω = 2 ground state with a U−O bond distance of 1.77 Å. The computed antisymmetric stretching σu frequency is 923 cm[-1] with a 16/18 isotope ratio of 1.0525 which compares with the experimental values of 915 cm[-1] and 1.0526, respectively. Calculations of the first adiabatic ionization energy gave the value 6.17 eV, which is 0.7 eV larger than the currently accepted experimental result. Reasons for this difference are suggested.
- Published
- 2001
35. On the low-lying singlet excited states of styrene: a theoretical contribution
- Author
-
Manuela Merchán, Björn O. Roos, Vicent Molina, and Per-Åke Malmqvist
- Subjects
chemistry.chemical_compound ,chemistry ,Photoisomerization ,Band gap ,Excited state ,General Physics and Astronomy ,Torsion (mechanics) ,Singlet state ,Physical and Theoretical Chemistry ,Atomic physics ,Methylene ,Potential energy ,Styrene - Abstract
The present contribution analyses the trans–cis photoisomerization mechanism of ethene and styrene on the singlet manifold. Within the framework of multiconfigurational second-order perturbation theory (CASPT2), the extended multistate approach (MS-CASPT2) is found to be flexible enough to describe energy hypersurfaces adiabatically. For ethene, torsion about the CC bond towards a perpendicular structure leads to a situation where the energy difference between the ground and the lowest excited state is still too large (2.5 eV) for efficient radiationless decay. However, the energy gap decreases to 0.4 eV when one of the methylene moieties is, in addition, pyramidalized from the twisted structure. A similar behaviour is obtained for styrene upon torsion-pyramidalization of the methylene subunit, with a computed energy gap of 0.2 eV. The intersection of the ground and excited states potential energy surfaces occurs by a slight additional pyramidalization from the optimal excited state structure. The actual barrier height on the S1 surface from the local planar minimum structure is estimated to be less than 0.8 eV.
- Published
- 2000
36. Dissociative recombination of HeH + . I. Rovibrational spectrum of HeH Rydberg states
- Author
-
Per-Åke Malmqvist and Wolfgang P. Kraemer
- Subjects
Chemistry ,Rotational–vibrational spectroscopy ,Electron ,State (functional analysis) ,Ion ,symbols.namesake ,Radiative transfer ,Rydberg formula ,symbols ,Physics::Atomic Physics ,Physical and Theoretical Chemistry ,Atomic physics ,Rydberg state ,Dissociative recombination - Abstract
The repulsive ground electronic state X2Σ+ of HeH is strongly coupled to the Rydberg states at small interatomic distances. Such large couplings also occur between some of the Rydberg states. HeH+ ions that capture an electron in a Rydberg state end up in separated He and H atoms by indirect predissociation. This paper presents a study of potential functions and pertinent matrix elements involving the lowest electronic states: the 2Σ+ states, X, A, C, and D, and the 2Π states B and E. Individual transition rates as well as total radiative and non-radiative lifetimes have been computed for the lowest vibrational and rotational levels.
- Published
- 1998
37. The multi-state CASPT2 method
- Author
-
Per-Åke Malmqvist, Luis Serrano-Andrés, Björn O. Roos, and James P. Finley
- Subjects
Multi state ,Ab initio multiple spawning ,Chemistry ,Avoided crossing ,Potential curves ,General Physics and Astronomy ,Perturbation (astronomy) ,Applied mathematics ,Physical and Theoretical Chemistry ,Atomic physics ,Electronic states - Abstract
An extension of the multiconfigurational second-order perturbation approach CASPT2 is suggested, where several electronic states are coupled at second order via an effective-Hamiltonian approach. The method has been implemented into the MOLCAS-4 program system, where it will replace the single-state CASPT2 program. The accuracy of the method is illustrated through calculations of the ionic-neutral avoided crossing in the potential curves for LiF and of the valence-Rydberg mixing in the V-state of the ethylene molecule.
- Published
- 1998
38. Franck-Condon factors for multidimensional harmonic oscillators
- Author
-
Niclas Forsberg and Per-Åke Malmqvist
- Subjects
Chemistry ,Mathematical analysis ,General Physics and Astronomy ,Recursion (computer science) ,Matrix multiplication ,LU decomposition ,law.invention ,Matrix (mathematics) ,Simple (abstract algebra) ,Normal mode ,law ,Quantum mechanics ,Physical and Theoretical Chemistry ,Wave function ,Harmonic oscillator - Abstract
We present a simple formula for the overlap integrals of two sets of multi-dimensional harmonic oscillators. The oscillators have in general different equilibrium points, force constants, and natural vibration modes. The formula expresses the overlap matrix in the one-dimensional case, 〈m′|n′′〉, as a so-called LU decomposition, 〈m′|n′′〉=〈0′|0′′〉 ∑ L mt U tn , where the summation index has a range 0≤t≤min(m,n), i.e., it is the matrix product of a lower-triangular matrix L with an upper-triangular U. These matrices are obtained from simple recursion formulae. This form is essentially retained in the multi-dimensional case. General matrix elements are obtained by exact and finite expressions, relating them to matrix elements over a single set of harmonic oscillator wave functions. We present test calculations with error estimates, also comparing with literature examples.
- Published
- 1998
39. Multiconfiguration perturbation theory with imaginary level shift
- Author
-
Niclas Forsberg and Per-Åke Malmqvist
- Subjects
Distortion (mathematics) ,Physics ,Singularity ,Quantum mechanics ,Quantum electrodynamics ,Excited state ,General Physics and Astronomy ,Gravitational singularity ,Function (mathematics) ,Physical and Theoretical Chemistry ,Perturbation theory ,Ground state ,Potential energy - Abstract
In multiconfigurational perturbation theory, so-called intruders may cause singularities in the potential energy functions, at geometries where an energy denominator becomes zero. When the singularities are weak, they may be successfully removed by level shift techniques. When applied to excited states, a small shift merely moves the singularity. A large shift may cause new divergencies, and too large shifts are unacceptable since the potential function is affected in regions further away from the singularities. This Letter presents an alternative which may be regarded as an imaginary shift. The singularities are not moved, but disappear completely. They are replaced by a small distortion of the potential function. Applications to the N 2 ground state, its A 3 / gE u + state, and the Cr 2 ground state show that the distortion caused by this procedure is small.
- Published
- 1997
40. On the use of a Hessian model function in molecular geometry optimizations
- Author
-
Gunnar Karlström, Anders Bernhardsson, Per-Åke Malmqvist, and Roland Lindh
- Subjects
Hessian matrix ,Physics ,Hessian equation ,Ab initio ,General Physics and Astronomy ,Function (mathematics) ,symbols.namesake ,Molecular geometry ,Computational chemistry ,Homogeneous space ,symbols ,Quasi-Newton method ,Applied mathematics ,Physical and Theoretical Chemistry ,Row - Abstract
When a molecular equilibrium geometry is determined by minimizing the energy by a quasi-Newton-Raphson method, the number of iterations required depends critically on the choice of an approximate molecular Hessian matrix. We find that a simple 15-parameter function of the nuclear positions gives a good choice for any molecule with atoms from the first three rows of the periodic table. This Hessian is used for ab initio geometry optimizations with the quasi-Newton-Raphson method, with or without update. The equilibrium geometries of 30 molecules, with a variety of sizes and symmetries, is obtained with the new scheme, which is shown to converge significantly faster than other methods.
- Published
- 1995
41. A theoretical study of the low-lying excited states of ozone
- Author
-
Per-Åke Malmqvist, Markus P. Fülscher, Piotr Borowski, and Björn O. Roos
- Subjects
Vibration ,chemistry.chemical_compound ,Ozone ,chemistry ,Excited state ,General Physics and Astronomy ,Molecule ,Physical and Theoretical Chemistry ,Atomic physics ,Ground state ,Dissociation (chemistry) ,Excitation ,Fock space - Abstract
A theoretical study has been performed on the five lowest excited states of the ozone molecule using multiconfigurational second-order perturbation theory (CASPT2). The predicted order of states is: 3A2 (T0 = 1.15 eV), 3B2 (T0 = 1.33 eV, 3B1 (T0 = 1.33 eV), 1A2 (T0 = 1.44 eV) and 1B1 (T0 = 1.88 eV). Corresponding experimental data are: 1.18, 1.30, 1.45, 1.58, and 2.05 eV, respectively. Equilibrium geometries, harmonic frequencies of symmetric vibrations, and vertical excitation energies are also reported. The dissociation limit De for the ground state of ozone is found to be 1.08 eV, in agreement with the experimental value (1.13 eV). The calculations make use of a modified Fock operator in the CASPT2 theory. Relative energies of states with a different number of open shells are substantially improved. The modified CASPT2 method was checked by calculating spectroscopic constants of the oxygen molecule.
- Published
- 1995
42. Bj�rn?s top ten
- Author
-
Per-Åke Malmqvist and Roland Lindh
- Subjects
Polymer science ,Computational chemistry ,Chemistry ,Theoretical chemistry ,Physical and Theoretical Chemistry - Published
- 2003
43. ChemInform Abstract: How to Select Active Space for Multiconfigurational Quantum Chemistry?
- Author
-
Per-Åke Malmqvist, Valera Veryazov, and Björn O. Roos
- Subjects
Algebra ,Set (abstract data type) ,Development (topology) ,Field (physics) ,Atomic orbital ,Chemistry ,General Medicine ,Complete active space ,Perturbation theory ,Quantum ,Quantum chemistry - Abstract
Bjorn Roos is one of the pioneers in the development and usage of multiconfigurational methods, in particular, the complete active space self-consistent field method and the perturbational complete active space perturbation theory through second order. To perform multiconfigurational calculations using these methods, a set of active orbitals must be selected, and the success of the methods depends on the choice of this set. This is not only sometimes easy but also sometimes difficult, especially for use of the more recent RASSCF and RASPT2 methods (which use a "restricted active space" rather than the complete one). Although an automated procedure for selecting the active orbitals would be a preferable solution, this does not seem feasible yet. An account of the problem is given, with examples and some approaches that usually work. (C) 2011 Wiley Periodicals, Inc. Int J Quantum Chem 111: 3329-3338, 2011 (Less)
- Published
- 2012
44. An ab initio study of aqueous Fe2+–Fe3+ electron transfer
- Author
-
Gunnar Karlström and Per-Åke Malmqvist
- Subjects
Electron transfer ,Hamiltonian matrix ,Ab initio quantum chemistry methods ,Chemistry ,Ab initio ,General Physics and Astronomy ,Complete active space ,Electron ,Physical and Theoretical Chemistry ,Atomic physics ,Wave function ,Basis set - Abstract
The tunneling rate for the transfer of an electron from an Fe2+ to an Fe3+ ion is determined by a Hamiltonian matrix element, which has been calculated for interionic distances in the range 5.10–7.22 A. The ions were either alone, or surrounded by water molecules. The calculations were made by computing the electronic wave function of the system by the complete active space self‐consistent field (CASSCF) method, and then evaluating the relevant matrix element for this wave function interacting with a symmetry image of itself, representing the final state. Basis set dependence, and in particular the effect of bond functions, was investigated, as well as the effect of removing some water molecules or replacing them with point charges. The results seem to indicate that earlier calculated values, which are frequently used, of the tunneling rate may be too large by as much as a factor of 10. If so, then the agreement between earlier calculated electron transfer rates and experimental data may be explained by c...
- Published
- 1992
45. Inclusion of dynamic ?-? polarization in ?-electronab initio calculations
- Author
-
Björn O. Roos and Per-Åke Malmqvist
- Subjects
Electronic correlation ,Chemistry ,Configuration interaction ,Polarization (waves) ,Molecular physics ,symbols.namesake ,Atomic orbital ,Computational chemistry ,symbols ,Chiropractics ,Complete active space ,Physical and Theoretical Chemistry ,Hamiltonian (quantum mechanics) ,Excitation ,Matrix method - Abstract
A method is presented, whereby dynamic σ-π polarization, i.e. the correlation effect expressed by simultaneous (σ-σ*, π-π*) excitations, can be approximately included in a multi-reference configuration interaction (MRCI) or multi-configurational self-consistent field (MC-SCF) calculation, without need to explicitly correlate the sigma orbitals. The method, which we call the capacitance matrix method, is based on the use of conventional one-electron integrals, from which a σ polarization potential (SPP) contribution is computed and added to the one- and two-electron Hamiltonian. In the present form, the method requires one parameter for each type of atom, and one for each type of bond. These parameters were adjusted to reproduce the dynamic σ-π polarization energy, computed by restricted multi-reference CI calculations, of a number of states of different hydrocarbons, and the agreement was within a few percent. Using the same parameters in CAS (Complete Active Space) SCF calculations of various states of benzene gives excitation energies, when SPP is included, which is comparable to those obtained by much more elaborate MRCI calculations.
- Published
- 1992
46. Inversion-vibration energies of CH3− and adiabatic electron affinity of CH3
- Author
-
Björn O. Roos, V. Špirko, Per-Åke Malmqvist, and Wolfgang P. Kraemer
- Subjects
Physics ,Electronic correlation ,Anharmonicity ,Methyl radical ,Configuration interaction ,Molecular physics ,Atomic and Molecular Physics, and Optics ,Ion ,chemistry.chemical_compound ,symbols.namesake ,Nuclear magnetic resonance ,chemistry ,Kinetic isotope effect ,symbols ,Physics::Chemical Physics ,Physical and Theoretical Chemistry ,Ground state ,Hamiltonian (quantum mechanics) ,Spectroscopy - Abstract
The inversion potential of the X 1 A 1 electronic ground state of the methyl anion CH 3 − is investigated at the SCF/single-reference configuration interaction ( SCF SR-CI ) and at the completeactive-space SCF/multireference CI ( CASSCF MR-CI ) levels of theory employing large Gaussian basis sets augmented by diffuse functions to represent the weakly bound carbon lone-pair electrons. These calculations, together with the corresponding out-of-plane bending potential of the CH 3 radical, demonstrate that the Born-Oppenheimer separation of the electronic and nuclear motions becomes invalid when moving along the inversion potential of CH 3 − ion by conventional methods, a quasi-diabatic representation of the full-dimensional potential energy hypersurface is evaluated applying standard SCF SR-CI calculations. From the resulting anharmonic potential function the vibration-rotation energies of the CH 3 − and CD 3 − isotopic variants are calculated using an improved version of the nonrigid inverter Hamiltonian approximation. Combining the present results with the previously determined vibrational potential function for the methyl radical CH 3 , the different stabilities of the CH 3 − and CD 3 − isotopes against autodetachment are discussed in terms of effective potentials for the inversion motion and the adiabatic electron affinity of CH 3 is determined as EA = 0.09 eV close to the experimental value of 0.08 ± 0.03 eV. The photoelectron spectrum of CH 3 − is calculated within the Franck-Condon approximation in good agreement with the experimental spectrum. Present predictions of the lower inversion-vibration energy levels and the corresponding rotation constants of the CH 3 − ion are expected to be reliable.
- Published
- 1991
47. A restricted active space (RAS) SCF study of the lifetime of theA 3? state of OH+
- Author
-
Manuela Merchán, Per-Åke Malmqvist, and Björn O. Roos
- Subjects
Atomic orbital ,Chemistry ,Computational chemistry ,Transition dipole moment ,Chiropractics ,Rotational spectroscopy ,Physical and Theoretical Chemistry ,Atomic physics ,Ionization energy ,Configuration interaction ,Bond energy ,Diatomic molecule ,Ion - Abstract
Restricted Active Space (RAS) SCF calculations have been performed of the potential curves for theX3Σ− andA3Π states of the OH+ ion and on the lifetime of thev=0−2 vibrational levels of theA state. The convergence of the transition moment integral as a function of the size of the active orbital space was used to select the active orbitals. The calculated value of thev=0 lifetime is 2.4 µs. An estimate of the errors remaining in the calculation leads to a final theoretical value of 2.7±0.1 µs. Computed bond distances and bond energies are 1.031 (1.029) A and 5.05 (5.01) eV, respectively, for theX state, and 1.137 (1.135) A and 1.57 eV, respectively, for theA state (experimental values within parenthesis).
- Published
- 1991
48. New relativistic atomic natural orbital basis sets for lanthanide atoms with applications to the Ce diatom and LuF3
- Author
-
Björn O. Roos, Per-Åke Malmqvist, Valera Veryazov, Roland Lindh, Per-Olof Widmark, and Antonio Carlos Borin
- Subjects
Chemistry ,Cerium ,Fluorine ,Lutetium ,Lanthanoid Series Elements ,STO-nG basis sets ,Ion ,Chemical bond ,Atomic orbital ,Excited state ,Thermodynamics ,Computer Simulation ,Physics::Atomic Physics ,Physical and Theoretical Chemistry ,Atomic physics ,Ionization energy ,Relativistic quantum chemistry ,Wave function ,Dimerization - Abstract
New basis sets of the atomic natural orbital (ANO) type have been developed for the lanthanide atoms La-Lu. The ANOs have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive ions, and the atom in an electric field. Scalar relativistic effects are included through the use of a Douglas-Kroll-Hess Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second-order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies and some excitation energies. Computed ionization energies have an accuracy better than 0.1 eV in most cases. Two molecular applications are included as illustration: the cerium diatom and the LuF3 molecule. In both cases it is shown that 4f orbitals are not involved in the chemical bond in contrast to an earlier claim for the latter molecule.
- Published
- 2008
49. The RASSCF, RASSI, and CASPT2 methods used on small molecules of astrophysical interest
- Author
-
Per-Åke Malmqvist
- Subjects
Matrix (mathematics) ,Theoretical physics ,Field (physics) ,Interstellar cloud ,Stellar atmosphere ,Complete active space ,Perturbation theory ,Wave function ,Quantum - Abstract
To a quantum chemist with no particular background in astrophysics or astronomy, a brief glance at journals and textbooks in these fields shows at least three areas where computational quantum chemistry has had a valuable impact: Interstellar cloud chemistry; stellar atmosphere modelling; and chemistry in extreme conditions, such as at the surface of a neutron star. The first two uses are particularly suitable, since standard methods are directly applicable. For such problems, good calculations of potential energy as well as expectation values and matrix elements of dipole and other operators appears to be in demand. Many electronic states may be involved, at least a broad range of problems involve fairly small molecules, often radicals, and conformation regions far from equilibrium. Such problems are addressed by three methods originated in our laboratory, and known by the acronyms RASSCF (Restricted Active Space Self-Consistent Field, Malmqvist et al. 1990), RASSI (RAS State Interaction) and CASPT2 (Complete Active Space Perturbation Theory to Second Order-Complete Active Space Perturbation Theory to Second Order, Andersson et al. 1990; Andersson et al. 1992).
- Published
- 2008
50. Exploring the Actinide—Actinide Bond: Theoretical Studies of the Chemical Bond in Ac2, Th2, Pa2, and U2
- Author
-
Laura Gagliardi, Björn O. Roos, and Per Åke Malmqvist
- Subjects
010405 organic chemistry ,Chemistry ,Bond ,General Chemistry ,General Medicine ,Actinide ,010402 general chemistry ,Triple bond ,01 natural sciences ,7. Clean energy ,Biochemistry ,Bond order ,Bent bond ,Catalysis ,0104 chemical sciences ,Bond length ,Colloid and Surface Chemistry ,Chemical bond ,Computational chemistry ,Sextuple bond ,ddc:540 ,Single bond ,Physical chemistry ,Bond energy - Abstract
Multiconfigurational quantum chemical methods (CASSCF/CASPT2) have been used to study the chemical bond in the actinide diatoms Ac2, Th2, Pa2, and U2. Scalar relativistic effects and spin-orbit coupling have been included in the calculations. In the Ac2 and Th2 diatoms the atomic 6d, 7s, and 7p orbitals are the significant contributors to the bond, while for the two heavier diatoms, the 5f orbitals become increasingly important. Ac2 is characterized by a double bond with a 3Sigmag-(0g+) ground state, a bond distance of 3.64. A, and a bond energy of 1.19 eV. Th2 has quadruple bond character with a 3Dg(1g) ground state. The bond distance is 2.76 A and the bond energy (D0) 3.28 eV. Pa2 is characterized by a quintuple bond with a 3Sigmag-(0g+) ground state. The bond distance is 2.37 A and the bond energy 4.00 eV. The uranium diatom has also a quintuple bond with a 7Og (8g) ground state, a bond distance of 2.43 A, and a bond energy of 1.15 eV. It is concluded that the strongest bound actinide diatom is Pa2, characterized by a well-developed quintuple bond.
- Published
- 2007
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