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We discuss the structural, electronic, and vibrational properties of an adsorbed CO monolayer in the presence of liquid water at room temperature, as obtained by ab initio molecular dynamics simulations. The water molecules at closest distance from the substrate form a bilayer, where they are coadsorbed at empty surface sites and with relatively rigid orientation, at the difference with the bulk liquid above. Coadsorbed water strengthens the back-bonding to CO, which leads to the red shift of about 40 cm–1. The synergy between the first bilayer and the bulk liquid induces further polarization of the CO bond, which changes the transition dipole and results in the CO intensity enhancement. It is also verified that no hydrogen bond is effectively present between CO and bulk water. These findings provide new insight into the physics of the surface/adsorbate/solvent interface, thus clarifying the experimental observation to be exploited for the design of improved catalysts.

Linear free energy scaling relationships (LFESRs) and volcano plots are routinely used to assess the performance of heterogeneous electrocatalysts and have only recently been concretely exploited in homogeneous catalysis. These tools efficiently compare and provide a global evaluation of catalyst performance while highlighting the limitations for a given reaction. In the framework of solid-state water oxidation, a minimal overpotential of 0.4 eV has been predicted on the basis of LFESRs. Considering the very different nature of homogeneous catalysts compared to solid-state systems, the validity of scaling relationships determined for the former cannot be assumed. To evaluate the global limitations of molecular O2evolution catalysts, LFESRs are established for all key intermediates for different metal (Mn, Co, Ru, Rh, Ir) and ligand (corrole and perfluoro-porphyrin) combinations assuming a mononuclear mechanism that proceeds through *–OH, *═O, and *–OOH intermediates. Our computations indicate that the LFESRs strongly depend on the choice of density functional. Using GMC-QDPT2 as a benchmark, strong scaling relationships between all intermediates are observed, but the relationships between *–OH and *═O significantly differ from those found in solid-state systems. Consequently, the shape of the molecular volcano plot changes drastically from its solid-state counterpart and shows a broad plateau at the top where the overpotential is nearly independent of the choice of catalyst. This plateau renders the performance of molecular catalysts extremely robust, but inhibits improvements by proceeding through alternative reaction mechanisms.

Large-scale computations of nuclear magnetic resonance (NMR) shifts for extended paramagnetic solids (pNMR) are reported using the highly efficient Gaussian-augmented plane-wave implementation of the CP2K code. Combining hyperfine couplings obtained with hybrid functionals with g-tensors and orbital shieldings computed using gradient-corrected functionals, contact, pseudocontact, and orbital-shift contributions to pNMR shifts are accessible. Due to the efficient and highly parallel performance of CP2K, a wide variety of materials with large unit cells can be studied with extended Gaussian basis sets. Validation of various approaches for the different contributions to pNMR shifts is done first for molecules in a large supercell in comparison with typical quantum-chemical codes. This is then extended to a detailed study of g-tensors for extended solid transition-metal fluorides and for a series of complex lithium vanadium phosphates. Finally, lithium pNMR shifts are computed for Li3V2(PO4)3, for which detailed experimental data are available. This has allowed an in-depth study of different approaches (e.g., full periodic versus incremental cluster computations of g-tensors and different functionals and basis sets for hyperfine computations) as well as a thorough analysis of the different contributions to the pNMR shifts. This study paves the way for a more-widespread computational treatment of NMR shifts for paramagnetic materials.

A local resolution-of-the-identity (LRI) approach is introduced in combination with the Gaussian and plane waves (GPW) scheme to enable large-scale Kohn–Sham density functional theory calculations. In GPW, the computational bottleneck is typically the description of the total charge density on real-space grids. Introducing the LRI approximation, the linear scaling of the GPW approach with respect to system size is retained, while the prefactor for the grid operations is reduced. The density fitting is an O(N) scaling process implemented by approximating the atomic pair densities by an expansion in one-center fit functions. The computational cost for the grid-based operations becomes negligible in LRIGPW. The self-consistent field iteration is up to 30 times faster for periodic systems dependent on the symmetry of the simulation cell and on the density of grid points. However, due to the overhead introduced by the local density fitting, single point calculations and complete molecular dynamics steps, including the calculation of the forces, are effectively accelerated by up to a factor of ∼10. The accuracy of LRIGPW is assessed for different systems and properties, showing that total energies, reaction energies, intramolecular and intermolecular structure parameters are well reproduced. LRIGPW yields also high quality results for extended condensed phase systems such as liquid water, ice XV, and molecular crystals.

Understanding molecule–metal interfaces is crucial for technologies such as molecular electronics, magnetism, and photovoltaic cells. However, due to the complex nature of the Au(111) surface, which possesses herringbone reconstruction, the interactions between molecules and the reconstructed Au(111) surface are still unclear. To fill this fundamental gap in the literature, we apply Density Functional Theory (DFT) to address the role of this reconstruction and more in general of the registry of the molecule and assembly with respect to the underlying metal on the adsorption strength and on the modification of the molecular properties. We find that the type of van der Waals schemes is important to get accurate herringbone reconstruction of the Au(111) surface. Adsorption simulations verify that the dominant contribution to the adsorption energy are dispersion forces, followed by the interaction of the cyano groups with the metal. The insertion of the Co atom further stabilizes the adsorption, due to the attractive interaction between Co and the surface. While changes in adsorption registry of the molecule lead to have weaker binding energy and different conformation of the molecule on the surface, adsorption energies, and structural parameters of the molecules on different surface domains are very similar. The formation of the monolayer and the geometrical configuration of the assembly are mainly driven by the molecule/molecule interactions. Experimental observation indicated that the metalation of pyrphyrin by coadsorption of Co atoms occurs slowly or upon annealing. In order to address these issues, we investigate the assembly and metalation process by means of ab initio molecular dynamics. We show the presence of intermediate states that hinder the process. Moreover, rearrangements within the monolayer are observed upon metalation, which are in agreement with experimental evidence.

We present an algorithm for computing the correlation energy in the random phase approximation (RPA) in a Gaussian basis requiring O(N3)operations and O(N2)memory. The method is based on the resolution of the identity (RI) with the overlap metric, a reformulation of RI-RPA in the Gaussian basis, imaginary time, and imaginary frequency integration techniques, and the use of sparse linear algebra. Additional memory reduction without extra computations can be achieved by an iterative scheme that overcomes the memory bottleneck of canonical RPA implementations. We report a massively parallel implementation that is the key for the application to large systems. Finally, cubic-scaling RPA is applied to a thousand water molecules using a correlation-consistent triple-ζ quality basis.

We determine and compare structural, dynamical, and electronic properties of liquid water at near ambient conditions through density-functional molecular dynamics simulations, when using either plane-wave or atomic-orbital basis sets. In both frameworks, the electronic structure and the atomic forces are self-consistently determined within the same theoretical scheme based on a nonlocal density functional accounting for van der Waals interactions. The overall properties of liquid water achieved within the two frameworks are in excellent agreement with each other. Thus, our study supports that implementations with plane-wave or atomic-orbital basis sets yield equivalent results and can be used indiscriminately in study of liquid water or aqueous solutions.

We present an implementation of G0W0and eigenvalue-self-consistent GW(evGW) in the Gaussian and plane waves scheme for molecules. We calculate the correlation self-energy for imaginary frequencies employing the resolution of the identity. The correlation self-energy for real frequencies is then evaluated by analytic continuation. This technique allows an efficient parallel implementation and application to systems with several hundreds of atoms. Various benchmark calculations are presented. In particular, the convergence with respect to the most important numerical parameters is assessed for the benzene molecule. Comparisons with respect to other G0W0implementations are reported for a set of molecules, while the performance of the method has been measured for water clusters containing up to 480 atoms in a cc-TZVP basis. Additionally, G0W0has been applied for studying the influence of the ligands on the gap of small CdSe nanoparticles. evGWhas been employed to calculate the HOMO–LUMO gaps of linear acenes, linear chains formed of connected benzene rings. Distinct differences between the closed and the open-shell (broken-symmetry) evGWHOMO–LUMO gaps for long acenes are found. In future experiments, a comparison of measured HOMO–LUMO gaps and our calculated evGWvalues may be helpful to determine the electronic ground state of long acenes.

We study the water oxidation mechanism of the cobalt aqua complex [Co(H2O)6]2+in a photocatalytic setup by means of density functional theory. Assuming a water-nucleophilic-attack or radical coupling mechanism, we investigate how the oxidation state and spin configuration change during the catalytic cycle. In addition, different ligand environments are employed by substituting a water ligand with a halide, pyridine, or derivative thereof. This allows exploration of the effect of such ligands on the frontier orbitals and the thermodynamics of the water oxidation process. Moreover, the thermodynamically most promising water oxidation catalyst can be identified by comparing the computed free energy profiles to the one of an “ideal catalyst”. Examination of such simple (hypothetical) water oxidation catalysts provides a basis for the derivation of design guidelines, which are highly sought for the development of efficient homogeneous water oxidation catalysts.

Catalytic activity is of pivotal relevance in enabling efficient and selective synthesis processes. Recently, covalent coupling reactions catalyzed by solid metal surfaces opened the rapidly evolving field of on-surface chemical synthesis. Tailored molecular precursors in conjunction with the catalytic activity of the metal substrate allow the synthesis of novel, technologically highly relevant materials such as atomically precise graphene nanoribbons. However, the reaction path on the metal substrate remains unclear in most cases, and the intriguing question is how a specific atomic configuration between reactant and catalyst controls the reaction processes. In this study, we cover the metal substrate with a monolayer of hexagonal boron nitride (h-BN), reducing the reactivity of the metal, and gain unique access to atomistic details during the activation of a polyphenylene precursor by sequential dehalogenation and the subsequent coupling to extended oligomers. We use scanning tunneling microscopy and density functional theory to reveal a reaction site anisotropy, induced by the registry mismatch between the precursor and the nanostructured h-BN monolayer.

Suitable templates to steer the formation of nanostructure arrays on surfaces are indispensable in nanoscience. Recently, atomically thin sp2-bonded layers such as graphene or boron nitride (BN) grown on metal supports have attracted considerable interest due to their potential geometric corrugation guiding the positioning of atoms, metallic clusters or molecules. Here, we demonstrate three specific functions of a geometrically smooth, but electronically corrugated, sp2/metal interface, namely, BN/Cu(111), qualifying it as a unique nanoscale template. As functional adsorbates we employed free-base porphine (2H–P), a prototype tetrapyrrole compound, and tetracyanoquinodimethane (TCNQ), a well-known electron acceptor. (i) The electronic moirons of the BN/Cu(111) interface trap both 2H–P and TCNQ, steering self-organized growth of arrays with extended molecular assemblies. (ii) We report an effective decoupling of the trapped molecules from the underlying metal support by the BN, which allows for a direct visualization of frontier orbitals by scanning tunneling microscopy (STM). (iii) The lateral molecular positioning in the superstructured surface determines the energetic level alignment; i.e., the energy of the frontier orbitals, and the electronic gap are tunable.

A novel method for including polarization effects within hybrid quantum mechanics/molecular mechanics (QM/MM) simulations of adsorbate-metal systems is presented. The interactions between adsorbate (QM) and metallic substrate (MM) are described at the MM level of theory. Induction effects are additionally accounted for by applying the image charge formulation. The charge distribution induced within the metallic substrate is modeled by a set of Gaussian charges (image charges) centered at the metal atoms. The image charges and the electrostatic response of the QM potential are determined self-consistently by imposing the constant-potential condition within the metal. The implementation is embedded in a highly efficient Gaussian and plane wave framework and is naturally suited for periodic systems. Even though the electronic properties of the metallic substrate are not taken into account explicitly, the augmented QM/MM scheme can reproduce characteristic polarization effects of the adsorbate. The method is assessed through the investigation of structural and electronic properties of benzene, nitrobenzene, thymine, and guanine on Au(111). The study of small water clusters adsorbed on Pt(111) is also reported in order to demonstrate that the approach provides a sizable correction of the MM-based interactions between adsorbate and substrate. Large-scale molecular dynamics (MD) simulations of a water film in contact with a Pt(111) surface show that the method is suitable for simulations of liquid/metal interfaces at reduced computational cost.

Despite recent progress in linear scaling (LS) density function theory (DFT), the computational cost of the existing LS methods remains too high for a widespread adoption at present. In this work, we exploit nonorthogonal localized molecular orbitals to develop a series of LS methods for molecular systems with a low computational overhead. High efficiency of the proposed methods is achieved with a new robust two-stage variational procedure or by replacing the optimization altogether with an accurate nonself-consistent approach. We demonstrate that, even for challenging condensed-phase systems, the implemented LS methods are capable of extending the range of accurate DFT simulations to molecular systems that are an order of magnitude larger than those previously treated.

Time-dependent density functional theory has become state-of-the-art for describing photophysical and photochemical processes in extended materials because of its affordable cost. The inclusion of exact exchange was shown to be essential for the correct description of the long-range asymptotics of electronic interactions and thus a well-balanced description of valence, Rydberg, and charge-transfer excitations. Several approaches for an efficient treatment of exact exchange have been established for the ground state, while implementations for excited-state properties are rare. Furthermore, the high computational costs required for excited-state properties in comparison to ground-state computations often hinder large-scale applications on periodic systems with hybrid functional accuracy. We therefore propose two approximate schemes for improving computational efficiency for the treatment of exact exchange. Within the auxiliary density matrix method (ADMM), exact exchange is estimated using a relatively small auxiliary basis and the introduced basis set incompleteness error is compensated by an exchange density functional correction term. Benchmark results for a test set of 35 molecules demonstrate that the mean absolute error introduced by ADMM is smaller than 0.3 pm for excited-state bond lengths and in the range of 0.02–0.04 eV for vertical excitation, adiabatic excitation, and fluorescence energies. Computational timings for a series of covalent-organic frameworks demonstrate that a speed-up of at least 1 order of magnitude can be achieved for excited-state geometry optimizations in comparison to conventional hybrid functionals. The second method is to use a semiempirical tight binding approximation for both Coulomb and exchange contributions to the excited-state kernel. This simplified Tamm–Dancoff approximation (sTDA) achieves an accuracy comparable to approximated hybrid density functional theory when referring to highly accurate coupled-cluster reference data. We find that excited-state bond lengths deviate by 1.1 pm on average and mean absolute errors in vertical excitation, adiabatic excitation, and fluorescence energies are in the range of 0.2–0.5 eV. In comparison to ADMM-approximated hybrid functional theory, sTDA accelerates the computation of broad-band excitation spectra by 1 order of magnitude, suggesting its potential use for large-scale screening purposes.

The second-order Møller–Plesset perturbation energy (MP2) and the Random Phase Approximation (RPA) correlation energy are increasingly popular post-Kohn–Sham correlation methods. Here, a novel algorithm based on a hybrid Gaussian and Plane Waves (GPW) approach with the resolution-of-identity (RI) approximation is developed for MP2, scaled opposite-spin MP2 (SOS-MP2), and direct-RPA (dRPA) correlation energies of finite and extended system. The key feature of the method is that the three center electron repulsion integrals (μν|P) necessary for the RI approximation are computed by direct integration between the products of Gaussian basis functions μνand the electrostatic potential arising from the RI fitting densities P. The electrostatic potential is obtained in a plane waves basis set after solving the Poisson equation in Fourier space. This scheme is highly efficient for condensed phase systems and offers a particularly easy way for parallel implementation. The RI approximation allows to speed up the MP2 energy calculations by a factor 10 to 15 compared to the canonical implementation but still requires O(N5) operations. On the other hand, the combination of RI with a Laplace approach in SOS-MP2 and an imaginary frequency integration in dRPA reduces the computational effort to O(N4) in both cases. In addition to that, our implementations have low memory requirements and display excellent parallel scalability up to tens of thousands of processes. Furthermore, exploiting graphics processing units (GPU), a further speedup by a factor ∼2 is observed compared to the standard only CPU implementations. In this way, RI-MP2, RI-SOS-MP2, and RI-dRPA calculations for condensed phase systems containing hundreds of atoms and thousands of basis functions can be performed within minutes employing a few hundred hybrid nodes. In order to validate the presented methods, various molecular crystals have been employed as benchmark systems to assess the performance, while solid LiH has been used to study the convergence with respect to the basis set and system size in the case of RI-MP2 and RI-dRPA.

The adsorption of carbon dioxide on CeO2(111) has been studied using density functional theory. At low coverage (1/9 monolayer), CO2is found to preferably adsorb in a monodentate configuration forming a carbonate species with a surface O atom. In this configuration, the CO2molecule is bent with an O–C–O angle of 129° and a remarkable elongation (to 1.27 Å) of the C–O bond length compared to the gas phase molecule, indicating a high degree of CO2activation. A similar activation is observed when the CO2molecule adsorbs as bidentate carbonate; however, this configuration is less stable. Linear configurations are found to adsorb very weakly at low coverage by physisorption. Increasing the coverage leads to a decrease of the stability of mono- and bidentate configurations which can be attributed to repulsive interactions between adjacent adsorbates and the limited capacity of the CeO2(111) surface to donate electrons to the adsorbates. In contrast, the binding energy of linearly adsorbed CO2is shown to be coverage independent. At coverages >1/4 monolayer, we have also addressed the stability of mixed configurations where monodentate, bidentate, and linear species are present simultaneously on the surface. The most stable configurations are found when 1/3 monolayer CO2is bound as monodentate species, and additional molecules are physisorbed forming partial layers of linear species. Analysis of the projected density of states has shown that the orbitals of linear species in the first partial layer lie at lower energies than the ones of the second partial layer suggesting stabilization of the former through interactions with preadsorbed monodentate species. These findings provide fundamental insight into the CO2adsorption mechanism on CeO2and potentially assist the design of new Ce-based materials for CO2catalysis.

A novel algorithm, based on a hybrid Gaussian and plane waves (GPW) approach, is developed for the canonical second-order Møller–Plesset perturbation energy (MP2) of finite and extended systems. The key aspect of the method is that the electron repulsion integrals (ia|λσ) are computed by direct integration between the products of Gaussian basis functions λσand the electrostatic potential arising from a given occupied-virtual pair density ia. The electrostatic potential is obtained in a plane waves basis set after solving the Poisson equation in Fourier space. In particular, for condensed phase systems, this scheme is highly efficient. Furthermore, our implementation has low memory requirements and displays excellent parallel scalability up to 100 000 processes. In this way, canonical MP2 calculations for condensed phase systems containing hundreds of atoms or more than 5000 basis functions can be performed within minutes, while systems up to 1000 atoms and 10 000 basis functions remain feasible. Solid LiH has been employed as a benchmark to study basis set and system size convergence. Lattice constants and cohesive energies of various molecular crystals have been studied with MP2 and double-hybrid functionals.

In this work, the applicability and performance of a linear scaling algorithm is investigated for three-dimensional condensed phase systems. A simple but robust approach based on the matrix sign function is employed together with a thresholding matrix multiplication that does not require a prescribed sparsity pattern. Semiempirical methods and density functional theory have been tested. We demonstrate that self-consistent calculations with 1 million atoms are feasible for simple systems. With this approach, the computational cost of the calculation depends strongly on basis set quality. In the current implementation, high quality calculations for dense systems are limited to a few hundred thousand atoms. We report on the sparsities of the involved matrices as obtained at convergence and for intermediate iterations. We investigate how determining the chemical potential impacts the computational cost for very large systems.

Born–Oppenheimer molecular dynamics simulations in combination with calculations of 1H, 7Li, and 15N NMR chemical shifts are used to characterize lithium imide structures at different temperatures. Indications of the onset of local disorder in the lithium sublattice, leading eventually to superionicity, are recognized already at low temperature (100 K). Between 100 and 400 K, a new structure could be stabilized, which presents features that are intermediate between the previously reported Fm3m̅and the Fd3m̅structures. The disorder in the Li positions is associated with the reorientation of the NH bonds, which preferentially point toward Li-vacant sites. Clear signatures of such structural rearrangements are visible in the simulated NMR spectra, where smoother profiles are associated with a reduced amount of Li interstitials and a higher occupation probability of the antifluorite sites.

Recent scanning tunneling microscopy (STM) experiments have shown that, by dosing liquid water to the bare hexagonal boron nitride (h-BN) nanomesh on Rh(111), water clusters can be observed in the nanomesh pores. With the present work, we intend to better understand the nature of the interaction of water with the nanomesh and give indications on the type of structures that have been observed. To this purpose, the corrugated h-BN monolayer on a 12 × 12 Rh(111) slab is calculated, and its structural and electronic properties are studied in some detail. Then the interaction of small water clusters adsorbed on h-BN, with and without the metal, is investigated. The simulation gives insight into the nature of the binding of h-BN to the metal, the role of the corrugation in trapping molecules, and the structure and distribution of the hydrogen bonds formed by the water molecules. Through simulated STM topography, the optimized structures are compared to the experimental results and the most probable configurations of water aggregates within the pore are identified. The results of our calculations suggest that the water aggregates producing the predominant STM images with three protrusions forming an almost equilateral triangle, with side length of about 0.46 nm, are most likely water hexamers.

The structure of {a±,a±,b∓}-type Coulombic systems is characterized by the effective ground-state density of the a-type particles, computed via nonrelativistic quantum mechanics without introduction of the Born-Oppenheimer approximation. A structural transition is observed when varying the relative mass of the a- and b-type particles, e.g., between atomic H- and molecular H2+. The particle-density profile indicates a molecular-type behavior for the positronium ion, Ps-. [ABSTRACT FROM AUTHOR]

No Abstract

Mit H eben, ohne H gewellt: Bei der Behandlung einer Monoschicht aus hexagonalem Bornitrid auf Rhodium mit Wasserstoffatomen verschwindet die h‐BN‐Oberflächenwellung (siehe Bild: blauer Bereich gewellt, oranger Bereich flach). Diese Veränderung der Oberflächentextur, eine Folge der Interkalation von Wasserstoffatomen, lässt sich rückgängig machen, indem die Wasserstoffatome durch Erhitzen auf etwa 600 K entfernt werden.

The calculation of Hartree−Fock exchange (HFX) is computationally demanding for large systems described with high-quality basis sets. In this work, we show that excellent performance and good accuracy can nevertheless be obtained if an auxiliary density matrix is employed for the HFX calculation. Several schemes to derive an auxiliary density matrix from a high-quality density matrix are discussed. Key to the accuracy of the auxiliary density matrix methods (ADMM) is the use of a correction based on standard generalized gradient approximations for HFX. ADMM integrates seamlessly in existing HFX codes and, in particular, can be employed in linear scaling implementations. Demonstrating the performance of the method, the effect of HFX on the structure of liquid water is investigated in detail using Born−Oppenheimer molecular dynamics simulations (300 ps) of a system of 64 molecules. Representative for large systems are calculations on a solvated protein (Rubredoxin), for which ADMM outperforms the corresponding standard HFX implementation by approximately a factor 20.

In dye-sensitized solar cells, three structurally similar dyes are commonly employed to sensitize anatase nanocrystals, namely, the cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II) dye (N3) and its twice deprotonated (N719) and completely deprotonated (N712) forms. Using density functional theory, several possible binding geometries of these dyes are identified on the anatase(101) surface. Computed relative energies show that protonation of the surface can strongly influence the relative stabilities of these configurations and could induce a conformational transition from double bidentate-bridged binding to mixed bidentate/monodentate binding. Attenuated total reflection (ATR)-IR experiments and computed vibrational spectra provide additional support for a protonation-dependent equilibrium between two different configurations. Furthermore, self-assembly in chains of hydrogen-bonded dye molecules seems structurally favorable on the anatase(101) surface; for enantiopure dyes, a packing density of 0.744/nm2could be achieved.

Hartree−Fock exchange with a truncated Coulomb operator has recently been discussed in the context of periodic plane-waves calculations [Spencer, J.; Alavi, A. Phys. Rev. B: SolidState, 2008, 77, 193110]. In this work, this approach is extended to Gaussian basis sets, leading to a stable and accurate procedure for evaluating Hartree−Fock exchange at the Γ-point. Furthermore, it has been found that standard hybrid functionals can be transformed into short-range functionals without loss of accuracy. The well-defined short-range nature of the truncated exchange operator can naturally be exploited in integral screening procedures and makes this approach interesting for both condensed phase and gas phase systems. The presented Hartree−Fock implementation is massively parallel and scales up to ten thousands of cores. This makes it feasible to perform highly accurate calculations on systems containing thousands of atoms or ten thousands of basis functions. The applicability of this scheme is demonstrated by calculating the cohesive energy of a LiH crystal close to the Hartree−Fock basis set limit and by performing an electronic structure calculation of a complete protein (rubredoxin) in solution with a large and flexible basis set.

In this study, we present a scheme for the evaluation of electron delocalization and conjugation efficiency in lineraly π-conjugated systems. The scheme, based on the natural bond orbital theory, allows monitoring the evolution of electron delocalization along an extended conjugation path as well as its response to chemical modification. The scheme presented is evaluated and illustrated by means of a computational investigation of π-conjugation in all-transpolyacetylene [PA; H(−CHCH)nH], polydiacetylene [PDA, H(−CCCHCH)nH], and polytriacetylene [PTA, H(−CCCHCHCC)nH] with up to 180 carbon atoms, all related by the number of ethynyl units incorporated in the chain. We are able to show that for short oligomers the incorporation of ethynyl spacers into the PA chain increases the π-delocalization energy, but, on the other hand, reduces the efficiency with which π-electron delocalization is promoted along the backbone. This explains the generally shorter effective conjugation lengths observed for the properties of the polyeneynes (PDA and PTA) relative to the polyenes (PA). It will also be shown that the reduced conjugation efficiency, within the NBO-based model presented in this work, can be related to the orbital interaction pattern along the π-conjugated chain. We will show that the orbital interaction energy pattern is characteristic for the type and the length of the backbone and may therefore serve as a descriptor for linearly π-conjugated chains.

Pyranosides with Nbenzyl2,3transoxazolidinone undergo anomerization from the β form to the α form even in the presence of a weak Lewis acid. Experimental evidence for endocleavage, the breaking of the bond between the pyranoxygen and anomeric carbon atoms, in the anomerization reactions was obtained. This unexpected phenomenon was investigated by quantum mechanical calculations, which found clear differences in the transition states between anomerized and nonanomerized substrates. The computations suggest that BF3induces endocleavage followed by rotation of the C1–C2 bond to give the α form via lowerenergy transition states. © WileyVCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009

Car–Parrinello molecular dynamics (CPMD) simulations are used to investigate the structural properties of 1 and 2 molal (m) CaCl 2aqueous solutions and, in particular, the radial distribution functions, coordination numbers, and dipole moments of water molecules in the first solvation shell. According to these simulations, the first solvation shell of the Ca 2+ion consists of six water molecules, that are characterized by an increased averaged dipole moment compared to that of bulk water, and a first-shell Ca−O radial distribution function peak at 2.39 Å. The results are compared to those of CPMD simulations of Ca 2+(no counterions), and no significant differences are found. This indicates that the homogeneous neutralizing background charge density implicitly included in simulations of non-neutral systems appropriately mimics the presence of the counterions (at least in terms of reproducing the solvation structure properties and for the box sizes considered). Classical molecular dynamics (MD) simulations of aqueous Ca 2+using varying box sizes confirm this suggestion. The CPMD simulations at 2 m concentration also reveal additional possibilities for the structural arrangement of water molecules and chloride ions around Ca 2+. In particular, they support the stability of Ca 2+-Cl −(contact) and Ca 2+-H 2O-Cl −(solvent-separated) ion pairs. In addition, the solvent-separated cation pair is found to occur in a deprotonated Ca 2+-OH −-Ca 2+form. The existence of such a species has, to our knowledge, never been invoked previously to account for experimental data on CaCl 2solutions.

Complete reaction pathways relevant to CO2hydrogenation by using a homogeneous ruthenium dihydride catalyst ([Ru(dmpe)2H2], dmpe=Me2PCH2CH2PMe2) have been investigated by ab initio metadynamics. This approach has allowed reaction intermediates to be identified and free‐energy profiles to be calculated, which provide new insights into the experimentally observed reaction pathway. Our simulations indicate that CO2insertion, which leads to the formation of formate complexes, proceeds by a concerted insertion mechanism. It is a rapid and direct process with a relatively low activation barrier, which is in agreement with experimental observations. Subsequent H2insertion into the formateRu complex, which leads to the formation of formic acid, instead occurs via an intermediate [Ru(η2‐H2)] complex in which the molecular hydrogen coordinates to the ruthenium center and interacts weakly with the formate group. This step has been identified as the rate‐limiting step. The reaction completes by hydrogen transfer from the [Ru(η2‐H2)] complex to the formate oxygen atom, which forms a dihydrogen‐bonded RuH⋅⋅⋅HO(CHO) complex. The activation energy for the H2insertion step is lower for the transisomer than for the cisisomer. A simple measure of the catalytic activity was proposed based on the structure of the transition state of the identified rate‐limiting step. From this measure, the relationship between catalysts with different ligands and their experimental catalytic activities can be explained.

We have studied reduction reactions for nitrogen fixation at Sellmann‐type model complexes with Car–Parrinello simulation techniques. These dinuclear complexes are especially designed to emulate the so‐called open‐side FeMoco model. The main result of this work shows that in order to obtain the reduced species several side reactions have to be suppressed. These involve partial dissociation of the chelate ligands and hydrogen atom transfer to the metal center. Working at low temperature turns out to be one necessary pre‐requisite in carrying out successful events. The successful events cannot be described by simple reaction coordinates. Complicated processes are involved during the initiation of the reaction. Our theoretical study emphasizes two experimental strategies which are likely to inhibit the side reactions. Clamping of the two metal fragments by a chelating phosphane ligand should prevent dissociation of the complex. Furthermore, introduction of tert‐butyl substituents could improve the solubility and should thus allow usage of a wider range of (mild) acids, reductants, and reaction conditions.

A general photochemical activation process of inert dinitrogen coordinated to two metal centers is presented on the basis of high‐level DFT and ab initio calculations. The central feature of this activation process is the occupation of an antibonding π* orbital upon electronic excitation from the singlet ground state S0to the first excited singlet state S1. Populating the antibonding LUMO weakens the triple bond of dinitrogen. After a vertical excitation, the excited complex may structurally relax in the S1state and approaches its minimum structure in the S1state. This excited‐state minimum structure features the dinitrogen bound in a diazenoid form, which exhibits a double bond and two lone pairs localized at the two nitrogen atoms, ready to be protonated. Reduction and de‐excitation then yield the corresponding diazene complex; its generation represents the essential step in a nitrogen fixation and reduction protocol. The consecutive process of excitation, protonation, and reduction may be rearranged in any experimentally appropriate order. The protons needed for the reaction from dinitrogen to diazene can be provided by the ligand sphere of the complexes, which contains sulfur atoms acting as proton acceptors. These protonated thiolate functionalities bring protons close to the dinitrogen moiety. Because protonation does not change the π*‐antibonding character of the LUMO, the universal and well‐directed character of the photochemical activation process makes it possible to protonate the dinitrogen complex beforeit is irradiated. The π*‐antibonding LUMO plays the central role in the activation process, since the diazenoid structure was obtained by excitation from various occupied orbitals as well as by a direct two‐electron reduction (without photochemical activation) of the complex; that is, the important bending of N2towards a diazenoid conformation can be achieved by populating the π*‐antibonding LUMO.

In this theoretical study on rhodium‐catalysed hydroformylation we examine an unmodified hydridorhodium(I) carbonyl system atogether with three variants modified by the model phosphane ligands PF3(system b), PH3(system c) and PMe3(system d), which show increasing basicity on the Tolman χparameter scale. The olefinic substrate for all systems is ethene. Based on the dissociative hydroformylation mechanism, static and dynamic quantum‐mechanical approaches are made for preequilibria and the whole catalytic cycle. Agreement with experimental results was achieved with regard to the predominance of phosphane monocoordination in systems b–d, different sensitivity of unmodified and modified systems towards hydrogen pressure and the early location of the rate‐determining step. Neither the catalytic cycle as a whole nor olefin insertion as an important selectivity‐determining step gives a clear picture of activity differences among a–d. However, the crucial first catalytic step, association of ethene to the active species [HRhL3] (L=CO, PR3), may play the key role in the experimentally observed higher activity of aand systems with less basic phosphane ligands modelled by b.

Abstract.: A new algorithm for density-functional-theory-based ab initio molecular dynamics simulations is presented. The Kohn–Sham orbitals are expanded in Gaussian-type functions and an augmented-plane-wave-type approach is used to represent the electronic density. This extends previous work of ours where the density was expanded only in plane waves. We describe the total density in a smooth extended part which we represent in plane waves as in our previous work and parts localised close to the nuclei which are expanded in Gaussians. Using this representation of the charge we show how the localised and extended part can be treated separately, achieving a computational cost for the calculation of the Kohn–Sham matrix that scales with the system size N as O(NlogN). Furthermore, we are able to reduce drastically the size of the plane-wave basis. In addition, we introduce a multiple-cutoff method that improves considerably the performance of this approach. Finally, we demonstrate with a series of numerical examples the accuracy and efficiency of the new algorithm, both for electronic structure calculations and for ab initio molecular dynamics simulations.

Minimum-energy structures of O2, CO, and NO iron–porphyrin (FeP) complexes, computed with the Car–Parrinello molecular dynamics, agree well with the available experimental data for synthetic heme models. The diatomic molecule induces a 0.3–0.4 Å displacement of the Fe atom out of the porphyrin nitrogen (Np) plane and a doming of the overall porphyrin ring. The energy of the iron–diatomic bond increases in the order Fe(SINGLE BOND)O2 (9 kcal/mol) < Fe(SINGLE BOND)CO (26 kcal/mol) < Fe(SINGLE BOND)NO (35 kcal/mol). The presence of an imidazole axial ligand increases the strength of the Fe(SINGLE BOND)O2 and Fe(SINGLE BOND)CO bonds (15 and 35 kcal/mol, respectively), with few structural changes with respect to the FeP(CO) and FeP(O2) complexes. In contrast, the imidazole ligand does not affect the energy of the Fe(SINGLE BOND)NO bond, but induces significant structural changes with respect to the FeP(NO) complex. Similar variations in the iron–imidazole bond with respect to the addition of CO, O2, and NO are also discussed. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 31–35, 1998

We present the ab initiomolecular dynamics code CPMD. This plane wave/pseudopotential based Kohn-Sham density functional code with a rich set of features is successfully applied to calculate static and dynamic properties for a wide range of molecules and materials. Its flexibility and high performance on many computer platforms make it an optimal tool for the study of liquids, surfaces, crystals as well as biomolecules.

Abstract.: Following an approach to density functional theory calculations based on the matrix representation of operators, we implemented a scheme as an alternative to traditional grid-based methods. These techniques allow integrals over exchange-correlation operators to be evaluated through matrix manipulations. Both local and gradient-corrected functionals can be treated in a similar way. After deriving all the required expressions, selected examples with various functionals are given.

The Random Phase Approximation (RPA), which represents the fifth rung of accuracy in Density Functional Theory (DFT), is made practical for large systems. Energies of condensed phase systems containing thousands of explicitly correlated electrons and 1500 atoms can now be computed in minutes and less than 1 h, respectively. GPU acceleration is employed for dense and sparse linear algebra, while communication is minimized by a judicious data layout. The performance of the algorithms, implemented in the widely used CP2K simulation package, has been investigated on hybrid Cray XC30 and XK7 architectures, up to 16,384 nodes. Our results emphasize the importance of good network performance, in addition to the availability of GPUs and generous on node memory. A new level of predictivity has thus become available for routine application in Monte Carlo and molecular dynamics simulations. [ABSTRACT FROM AUTHOR]

Abstract: Efficient Monte Carlo algorithms are combined with the Quickstep energy routines of CP2K to develop a program that allows for Monte Carlo simulations in the canonical, isobaric-isothermal, and Gibbs ensembles using a first principles description of the physical system. Configurational-bias Monte Carlo techniques and pre-biasing using an inexpensive approximate potential are employed to increase the sampling efficiency and to reduce the frequency of expensive ab initio energy evaluations. The new Monte Carlo program has been validated through extensive comparison with molecular dynamics simulations using the programs CPMD and CP2K. Preliminary results for the vapor–liquid coexistence properties () of water using the Becke–Lee–Yang–Parr exchange and correlation energy functionals, a triple-zeta valence basis set augmented with two sets of d-type or p-type polarization functions, and Goedecker–Teter–Hutter pseudopotentials are presented. The preliminary results indicate that this description of water leads to an underestimation of the saturated liquid density and heat of vaporization and, correspondingly, an overestimation of the saturated vapor pressure. [Copyright &y& Elsevier]

Abstract: We present the Gaussian and plane waves (GPW) method and its implementation in Quickstep which is part of the freely available program package CP2K. The GPW method allows for accurate density functional calculations in gas and condensed phases and can be effectively used for molecular dynamics simulations. We show how derivatives of the GPW energy functional, namely ionic forces and the Kohn–Sham matrix, can be computed in a consistent way. The computational cost of computing the total energy and the Kohn–Sham matrix is scaling linearly with the system size, even for condensed phase systems of just a few tens of atoms. The efficiency of the method allows for the use of large Gaussian basis sets for systems up to 3000 atoms, and we illustrate the accuracy of the method for various basis sets in gas and condensed phases. Agreement with basis set free calculations for single molecules and plane wave based calculations in the condensed phase is excellent. Wave function optimisation with the orbital transformation technique leads to good parallel performance, and outperforms traditional diagonalisation methods. Energy conserving Born–Oppenheimer dynamics can be performed, and a highly efficient scheme is obtained using an extrapolation of the density matrix. We illustrate these findings with calculations using commodity PCs as well as supercomputers. [Copyright &y& Elsevier]