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La energía fotovoltaica a partir de materiales orgánicos representa una opción muy atractiva, en términos de costo y flexibilidad, para generar electricidad. Sin embargo, las bajas eficiencias alcanzadas hasta ahora limitan la producción de dispositivos fotovoltaicos a escala comercial. En el intento de superar las bajas eficiencias hoy en día se dedican considerables esfuerzos en la investigación y el diseño de materiales orgánicos así como de arquitecturas de dispositivos más eficientes. En particular esta tesis doctoral presenta un trabajo de carácter fundamental sobre los procesos de fotoexcitación en dispositivos optoelectrónicos orgánicos, donde se usan química teórica y modelización computacional para estudiar las propiedades de los materiales a gran escala y predecir tecnologías energéticas más eficientes. Aquí se proponen nuevos materiales con aplicaciones prometedoras en el campo de dispositivos optolectrónicos orgánicos. Se presta especial atención a los procesos de transferencia de electrones en sistemas grandes y a la predicción de mecanismos no radiativos que, como se ha demostrado anteriormente a través de modelización computacional, contribuyen a la disminución de la eficiencia de los dispositivos. This dissertation concerns fundamental research on the photo-excitation processes in organic optoelectronics, where theoretical chemistry and computational modelling are used in order to investigate large scale material properties and predict more efficient energy technologies. Chapter 1 opens the thesis with the fundamentals of electronic and optoelectronic processes in organic photovoltaics. It covers different device architectures from which donor:acceptor (D:A) bulk heterojunctions (BHJs) are highlighted. This Chapter also describes some of the current challenges in terms of power conversion efficiency. These are the charge transfer exciton binding energy, the presence of non-radiative decay channels and the device morphology. Throughout this thesis, the properties of electronically excited state are studied in great detail while the morphology is discussed briefly. Chapter 2 outlines the general objective of this dissertation, namely to study theoretically and computationally the microscopic processes that underly the photovoltaic mechanisms at large scale. The specific objectives are to improve the technical implementation of the discrete reaction field (DRF) method in the Amsterdam Density Functional (ADF) modelling suite, to study charge transfer and charge separation processes such as those in large D:A BHJs and to explore the potential energy surfaces (PESs) of representative optoelectronic materials. Chapter 3 describes the electronic structure methods, wave function and electron density based methods, used in this thesis. Single- and multi-determinant wave function methods are described first, followed by density functional theory (DFT). DRF, which is widely used in the modelling of polarizable environments, is also introduced. In Chapters 4 to 6, DFT is used to study the excited state properties of photovoltaic materials. In Chapter 7, in addition to DFT, complete active space self-consistent field (CASSCF) and complete active space second order perturbation theory (CASPT2) methods are used to determine non-radiative decay paths in optoelectronic materials. Chapter 4 presents technical work carried out at the company Software for Che- mistry & Materials (SCM), involving an extended implementation of default input parameters required to perform DFT/DRF calculations with ADF. This implementation enables the accurate description of electrostatic potentials and dipole-dipole interactions in large system. The DRF input script for ADF is provided as supplementary information. Chapter 5 reports a calibration of exchange-correlation functionals for charge transfer states. It shows the importance of including long-range exchange-correlation functionals when studying intra- and inter-molecular charge transfer states. The influence of the environment on the charge transfer is included via DRF. The computing timings are also included to show that DRF models the polarizable environment very well, without significant additional computing time. Such a calibration turns out to be very useful when computing the excited state properties of polymer:fullerene derivative BHJs, as discussed in Chapter 6. Chapter 6 provides a theoretical study of the charge transfer exciton binding energy in semiconductor materials for BHJs. It covers four oligomers (PEO-PPV, PTFB, PTB7 and PTB7-Th), two fullerene derivatives ([60]PCBM and [70]PCBM) and eight oligomer:fullerene derivative BHJs (all the possible combinations between the four oligomers and the two fullerene derivatives). The absorption properties of single oligomers and fullerene derivatives and the excited state properties of oligomers:fullerene derivative BHJs are studied. Time-dependent density functional theory (TD-DFT) and DRF in a quantum mechanics/molecular mechanics (QM/MM) framework are used in order to compute the exciton binding energy from charge transfer and charge separated states. The results suggest that donor-acceptor (D-A) type oligomers (PTFB, PTB7 and PTB7-Th) perform better than the highly polarizable donor type oligomer PEO-PPV when blended to [60]PCBM or [70]PCBM. In addition, predicted charge transfer exciton binding energies are lower when D-A-type oligomers are blended with [70]PCBM. Mutual polarization effects from QM and MM regions on the exciton binding energy play a crucial role in the charge separation process. Chapter 7 describes an ab initio quantum chemistry study of luminescence in \pi -conjugated compounds with applications to optoelectronics. The non-radiative decay mechanisms of the distyrylbenzene cyano-substituted (DCS) family are discussed. This family comprises 33 compounds and is classified into two groups, \alpha - and \beta -compounds, where \alpha and \beta refer to the position of the cyano groups with respect to the central ring on the DSC backbone. The emissive character largely depends on the position of the cyano substituents. \beta -compounds, according to the experimental fluorescence quantum yields and computed non-radiative decay rates, tend to be more emissive than \alpha -compounds, with a few exceptions. A simplified strategy to explore the PESs and predict differences in the non-radiative channels, activated by a conical intersection between ground and excited states, is presented. From this strategy two energy descriptors are defined. These are the energy difference between ground and excited states in the Franck-Condon region, namely the absorption energy, and the energy difference between ground and excited states in the pyramidalization region, which is the region immediately before reaching the conical intersection. A good correlation between the non-radiative decay rates and the two energy descriptors is found. Chapter 8 suggests two ways for continued studies of photo-excitation processes in organic photovoltaics, based on the implementation of DFT/DRF energy gradients for the ground and excited states as well as electronic coupling and decay rates from non-orthogonal configuration interaction calculations. Chapter 9 closes the thesis with general conclusions and an outlook for large scale modelling research on next generation organic optoelectronics.

The full configuration interaction (FCI) study of the singlets vertical spectrum of the neutral beryllium trimer has been performed using atomic natural orbitals [3s2p1d] basis set. The FCI triangular equilibrium structure of the ground state has been used to calculate the FCI vertical excitation energies up to 4.8 eV. The FCI vertical ionization potential for the same geometry and basis set amounts to 7.6292 eV. The FCI dipole and quadrupole transition moments from the ground state are reported as well. The FCI electric quadrupole moment of the X (3)A(1) (') ground state has been also calculated with the same basis set (Theta(zz)=-2.6461 a.u., Theta(xx)=Theta(yy)=-1/2Theta(zz)). Twelve of the 19 calculated excited singlets are doubly excited states. Most of the states have large multiconfigurational character. These results provide benchmark values for electronic correlation multireference methods. (4ex6MO)CAS-SDCI values for the same energies and properties are also reported.

The doublet potential energy surfaces involved in the decomposition of the nitromethane radical anion (CH(3)NO(2) (-)) have been studied by using the multistate extension of the multiconfigurational second-order perturbation method (MS-CASPT2) in conjunction with large atomic natural orbital-type basis sets. A very low energy barrier is found for the decomposition reaction: CH(3)NO(2) (-)--[CH(3)NO(2)](-)--CH(3)+NO(2) (-). No evidence has been obtained on the existence of an isomerization channel leading to the initial formation of the methylnitrite anion (CH(3)ONO(-)) which, in a subsequent reaction, would yield nitric oxide (NO). In contrast, it is suggested that NO is formed through the bimolecular reaction: CH(3)+NO(2) (-)--[CH(3)O-N-O](-)--CH(3)O(-)+NO. In particular, the CASSCF/MS-CASPT2 results indicate that the methylnitrite radical anion CH(3)ONO(-) does not represent a minimum energy structure, as concluded by using density functional theory (DFT) methodologies. The inverse symmetry breaking effect present in DFT is demonstrated to be responsible for such erroneous prediction.

Triplet energy transfer (TET) from aromatic donors to 1,3,5,7-cyclooctatetraene (COT) is an extreme case of "nonvertical" behavior, where the transfer rate for low-energy donors is considerably faster than that predicted for a thermally activated (Arrhenius) process. To explain the anomalous TET of COT and other molecules, a new theoretical model based on transition state theory for nonadiabatic processes is proposed here, which makes use of the adiabatic potential energy surfaces (PES) of reactants and products, as computed from high-level quantum mechanical methods, and a nonadiabatic transfer rate constant. It is shown that the rate of transfer depends on a geometrical distortion parameter gamma=(2g(2)/kappa(1))(1/2) in which g stands for the norm of the energy gradient in the PES of the acceptor triplet state and kappa(1) is a combination of vibrational force constants of the ground-state acceptor in the gradient direction. The application of the model to existing experimental data for the triplet energy transfer reaction to COT from a series of pi,pi(*) triplet donors, provides a detailed interpretation of the parameters that determine the transfer rate constant. In addition, the model shows that the observed decrease of the acceptor electronic excitation energy is due to thermal activation of C=C bond stretchings and C-C bond torsions, which collectively change the ground-state COT bent conformation (D(2d)) toward a planar triplet state (D(8h)).

Layered double hydroxides (LDHs) are the leitmotiv of this dissertation. Contradicting the assertion that “any past was better”, LDHs have been continuously revisited from the middle of the twentieth century, and represent an excellent example of the never-ending beauty of Chemistry. New synthetic perspectives are giving a new impetus to LDH chemistry, which among hybrid materials, are finding their heyday. This is resulting in novel materials and also paving the way for new fundamental and practical insights. This dissertation is focused on magnetic LDHs, and for the sake of clarity it is organized in three main parts: we will move from basic synthetic and physical aspects of LDHs, through the preparation of multifunctional hybrids that respond to external stimuli, and towards some of the most innovative applications in energy storage and spintronics. Due to the wide number of techniques employed, and to facilitate the reading, we have included the experimental procedures and the specific instrumentation at the end of each Chapter. The first part deals with the chemical design, synthesis, exfoliation and physical characterization of new magnetic layered double hydroxides. Chapter 2 provides an in-depth insight into the synthesis of highly crystalline, hexagonal-shaped NiFe–LDH with tunable composition, a kind of LDH that up to now had been elusive. In addition, exfoliation of this magnetic LDH into unilamellar nanosheets has been achieved for the very first time. Chapter 3 is devoted to the physical properties of these new layered materials, unveiling their controllable magnetic behaviour, and shedding light on the cation order within the layers, elucidating the origin of the spin-glass behaviour in this sort of materials. Concluding this part, Chapter 4 addresses the profound effect exerted by extrinsic impurities in the magnetic behaviour of LDHs, highlighting the enormous possibilities offered by these magnetic materials. The second part explores the preparation of new hybrid magnetic multilayers based on LDHs, with special emphasis on the control of the magnetic properties of LDHs by means of an external stimulus. In this sense, Chapter 5 and 6 focuses on the physical effect induced by the intercalation of a macrocyclic phthalocyanine and porphyrins in the interlamellar space provided by magnetic LDHs, respectively. Moreover, the synthesis and characterization of novel stimuli responsive hybrid materials has been surveyed in this section, showing the very first examples of the reversible switching of the magnetic properties in LDH-based materials. Along this front, Chapter 7 deals with the photo-switching in a hybrid material made of azobenzene intercalated LDHs, whilst Chapter 8 examines the reversible thermo-responsive switching of a thermochromic-LDH hybrid. Finally, the third part is devoted to the applications of several LDH-based materials in different fields of utmost importance, such as energy storage and conversion or spintronics. On the one hand, in Chapter 9 we have studied the intrinsic electrocatalytic properties towards water oxidation of NiFe–LDHs. We have also investigated the effect of their hybridization with graphene showing an excellent behaviour as supercapacitors and water oxidation electrocatalysts. On the other hand, in Chapter 10 we have reported the use of hybrid NiFe–LDHs as catalytic nanoreactors for the in-situ synthesis of novel magnetic nanocomposites formed by FeNi3 nanoparticles embedded in carbon nanoforms. These hierarchical nanocomposites can be used as precursors for the low-temperature isolation of nanocarbons, ranging from carbon nanoonions to graphene. Furthermore, we have tested these nanocomposites as electrode materials for supercapacitors, obtaining high values of specific capacitance and excellent rate capabilities. In the end, we have explored the application of the previously synthesized graphene–FeNi3 nanocomposites in the field of spintronics. Concretely, Chapter 11 describes the magnetotransport properties of these hybrids, showing striking crossover behaviour from negative to positive magnetoresistance, arising from the correlation of the physical properties of its initial constituents in a synergistic way.

En el campo de la electrónica tradicional de semiconductores, la detección de cambios en la corriente eléctrica bajo la influencia de estímulos externos es el principio operacional de los dispositivos fabricados. La manipulación activa del grado de libertad de espín de los portadores de carga, sumado al control sobre la carga eléctrica, es el tema central que concierne a un campo de investigación relativamente joven denominado espintrónica. En el área de la nanotecnología, la espintrónica es una de los campos de investigación más emergentes, suponiendo una rama muy activa dentro del nanomagnetismo. El origen de esta área de conocimiento se puede situar en 1982 con el trabajo de Albert Fert y Peter Grünberg referido a la magnetoresistencia gigante (GMR) de multicapas metálicas. Por esta razón dichos investigadores fueron galardonados con el premio Nobel de física en 2007. La espintrónica ha supuesto un gran impacto en nuestro día a día mediante el desarrollo de sistemas de hardware para el almacenamiento de información y sensores magnéticos, entre otras aplicaciones. Hasta la fecha, este campo de investigación ha usado principalmente metales y semiconductores inorgánicos. Sin embargo, el rápido desarrollo experimentado en los campos de la electrónica molecular y el magnetismo molecular en la última década, junto con el progresivo desplazamiento hacia un escenario en la escala nanométrica, han propiciado las condiciones adecuadas para el surgimiento de una nueva área: la Espintrónica Molecular. De la misma manera que se hizo para la electrónica molecular, este nuevo campo de investigación se ha dividido en dos áreas principales: i) espintrónica basada en moléculas o espintrónica orgánica más comúnmente denominada por la comunidad física, ii) espintrónica unimolecular. Los temas tratados en la presente tesis se centran en la primera área de conocimiento. En este contexto, el principal objetivo es el diseño y fabricación de dispositivos espintrónicos innovadores y de menor coste mediante la utilización de materiales moleculares (más comúnmente denominados materiales orgánicos). El manuscrito de la tesis se encuentra dividido en dos partes. - Parte I: La primera parte de este manuscrito versa sobre la caracterización física de películas delgadas de análogos de azul de Prusia (PBA). Estos materiales basados en moléculas presentan un gran interés desde el punto de vista del control de sus propiedades por dos razones principales. En primer lugar, los PBAs son preparados mediante procesos sintéticos de baja complejidad. En segunda instancia, el diseño de sus propiedades magnéticas, tales como su ordenamiento ferromagnético, puede ser fácilmente alcanzado mediante la correcta selección de las fuentes de espín. Dos tipos de estructuras cristalinas pueden ser descritas para los PBA: i) AIMAII[MBIII(CN)6] siendo MA y MB iones de metales de transición y A un cation alcalino ocupando de forma alterna sitios intersticiales de la red; ii) AIMAII[MBIII(CN)6]2/3•zH2O conteniendo moléculas de agua coordinadas y sin coordinar. Cada tipo de estructura va a proveer de diferentes funcionalidades magnéticas y ópticas al material. En este sentido, es larga la lista de multifuncionalidades reportadas en los PBAs entre las cuales pueden destacarse: i) transición de fase por transferencia de carga, ii) fotomagnetismo, iii) piezomagnetismo, iv) second harmonic generation (SHG) y magnetization induced SHG (MSHG), v) coexistencia de ferroelectricidad y ferromagnetismo, vi) magnetismo sensible a la humedad, vii) Spin-iónica. Estos materiales magnéticos basados en moléculas pueden considerarse como potenciales candidatos para su aplicación en estructuras espintrónicas moleculares. En este sentido, el objetivo de esta tesis es desarrollar un amplio marco de conocimiento referido a las propiedades de los PBA al ser depositados en forma de películas delgadas, para lo cual se empleó la deposición electroquímica. La motivación para emplear esta técnica de crecimiento frente a otras, tales como el layer by layer (LbL), está suficientemente argumentada. A diferencia del método electroquímico, el LbL no necesita de sustratos conductores para el crecimiento de las capas delgadas. A pesar de esta ventaja, la técnica de LbL presenta una importante cantidad de inconvenientes. Para la fabricación de capas homogéneas se requiere un proceso secuencial caracterizado por un número elevado de ciclos de crecimiento. Esto deriva en un consumo importante de tiempo para la obtención del sistema final, lo cual dificulta el control sobre las condiciones de crecimiento. A su vez muchos parámetros deben de ser fijados para la obtención de películas delgadas reproducibles en términos de morfología y composición (y en consecuencia, propiedades magnéticas). Por el contrario, la deposición electroquímica es un método rápido y reproducible para la fabricación de capas delgadas homogéneas de PBAs sobre sustratos con áreas grandes. Mediante esta estrategia, un alto grado de control puede ser alcanzado en la referente la morfología, composición y propiedades magnéticas. El estudio llevado a cabo sobre los sistemas de PBA en esta primera parte de la tesis, está reflejado en cinco capítulos. El primero de ellos presenta una introducción a los conceptos fundamentales y técnicas experimentales correspondientes a esta parte de la tesis. En el capítulo 2 del manuscrito, se presentó una caracterización física completa de películas delgadas del PBA Cr5.5(CN)12 • 11.5 H2O en función del grosor de la capa. Cuatro grosores fueron elegidos para este estudio siendo de 1500 nm, 450nm, 250 nm y 80 nm. Este rango de grosores permitió evaluar de una manera completa posibles variaciones en las propiedades del material a medida que el proceso de fabricación evolucionaba hacia valores de este parámetro con interés para posibles aplicaciones. El estudio de IR-ATR llevado a cabo sobre estos sistemas muestra que, independientemente del grosor, estas películas delgadas sufren una oxidación completa como consecuencia de la exposición al aire. Dicho material se ha designado como CrCr a lo largo de toda la discusión de esta tesis con el fin de simplificar la notación. Este sistema presenta un comportamiento ferrimagnético, con una alta temperatura crítica (Tc) de 240 K, exhibiendo una alta estabilidad química y propiedades magnéticas controlables. En consecuencia, este PBA se plantea como uno de los candidatos más prometedores para su aplicación en espintrónica molecular. Es interesante remarcar que en este estudio se ha podido comprobar que al reducir el grosor de las capas, las propiedades magnéticas se ven considerablemente mejoradas (ciclos de histéresis con mayores campos coercitivos y forma cuadrada definida). En este capítulo de la tesis, el primer estudio de efecto Kerr magneto-óptico (MOKE) sobre películas delgadas de este PBA ha sido extensamente descrito. En este sentido, se ha demostrado que la magnetometría MOKE es una técnica potente para el estudio de las propiedades magnéticas de capas delgadas de estos materiales magnéticos basados en moléculas. Dichos materiales ofrecen una respuesta magneto-óptica excepcional debido a la alta intensidad de luz reflejada por su superficie transparente, aportando evidencias claras de efectos de tamaño de grano en el registro de la medida. En este capítulo, se introdujo un modelo para entender el origen de la sensibilidad superficial del MOKE en estos materiales transparentes. Dicho modelo fue formulado en base a consideraciones ópticas en el contexto de estudios elipsométricos. La naturaleza superficial de la información magnética adquirida mediante MOKE permitió el registro de los procesos de reversión de la magnetización que tienen lugar en las capas superiores de estas películas delgadas de CrCr. Esta región interfacial es de gran interés, teniendo en cuenta su posible uso como electrodos inyectores de espín, puesto que dicha región definirá la calidad de las interfases en el dispositivo espintrónico resultante. Un estudio de Microscopía de Fuerza Magnética a baja temperatura (LT-MFM) se ha realizado sobre los films de CrCr de 80 nm de grosor. Los resultados de esta caracterización indican que el alto valor de campo coercitivo determinado por MOKE para las capas delgadas de este PBA son debidas a un efecto de tamaño de las nanopartículas en la superficie. Esta información no pudo ser obtenida mediante SQUID, siendo esta una caracterización referida al volumen completo del material, sin la posibilidad de alcanzar una sensibilidad superficial. Sumado a todo lo descrito hasta ahora, se pudo comprobar que la magnetometría MOKE permite el registro de ciclos de histéresis del CrCr a temperaturas relativamente altas (justo por debajo de la Tc), lo cual resulta de gran interés en términos de futuras aplicaciones. De hecho, el estudio desarrollado en este capítulo, mostró la capacidad de obtener ciclos de histéresis a temperaturas próximas a la Tc para capas delgadas de este PBA ferrimagnético con grosores ultra-delgados de 40 nm. Con el fin de evaluar la validez de las conclusiones extraídas de las medidas MOKE sobre el CrCr, se abordó un estudio paralelo sobre otra familia de PBA constituida por el sistema ternario (FeIIxCrII1-x)3 [CrIII(CN)6]2•z H2O, al que se denominó FeCrCr a lo largo de la discusión del manuscrito. Los estudios realizados, confirmaron las propiedades determinadas mediante MOKE para las capas delgadas de CrCr y su relación con respecto a la información proporcionada por el SQUID, otorgando universalidad a las conclusiones extraídas. Las habilidades únicas del MOKE para estudiar las propiedades magnéticas de PBAs, se testó de igual forma sobre capas delgadas de la familia del PBA Fe3[Cr(CN)6]2•15 H2O, denominado FeCr a lo largo de la discusión. Este trabajo se encuentra extensamente descrito en el capítulo 3 de este texto. En películas delgadas de FeCr se detectó una discrepancia entre la temperatura crítica determinada mediante MOKE (Tc=60-70 K) y la hallada mediante SQUID (Tc=20 K) siendo esta última la habitualmente reportada en literatura para este material. Esta anomalía en la determinación de la Tc se explicó en base a la presencia de una fase de óxido de hierro amorfa. Dicha fase aparecería en el entorno del electrolito e introduciéndose en la estructura del PBA durante el proceso de electrodeposición de las capas. Los estudios de Raman sobre las películas delgadas de este material descartaron cualquier tipo de fenómeno fotomagnético como responsable de esta anomalía en la Tc. Esta información llevó a considerar un origen externo de esta anomalía, resultado de la incorporación de una fase química extra, la cual podía ser detectada por el MOKE pero no por medio de SQUID. Observando la evolución del electrolito, fue posible determinar la aparición de un material precipitado que se aisló y caracterizó tanto morfológicamente como en sus propiedades magnéticas. De esta manera se pudo identificar como una fase amorfa de óxido de hierro. A partir del análisis de TEM de alícuotas del electrolito, las mismas estructuras amorfas fueron detectadas. Con esta información fue posible concluir que en el entorno químico, en el cual la electropolimerización de las capas delgadas tiene lugar, estructuras basadas en hierro aparecen espontáneamente. Estas impurezas son incluidas en la estructura del PBA durante la electrodeposición, acumulándose principalmente en la superficie de la película delgada. Esta es la razón por la cual la señal magnética de dicha fase extra es sólo detectada mediante MOKE. Esta fase extra sólo pudo ser medida para tiempos de deposición altos (1000 segundos). Para tiempos menores (

A new approach to carry out molecular dynamics simulations of chemical reactions in solution using combined density functional theory/molecular mechanics potentials is presented. We focus our attention on the analysis of reactive trajectories, dynamic solvent effects and transmission coefficient rather than on the evaluation of free energy which is another important topic that will be examined elsewhere. In a previous paper we have described the generalities of this hybrid molecular dynamics method and it has been employed to investigate low energy barrier proton transfer process in water. The study of processes with activation energies larger than a few kT requires the use of specific techniques adapted to “rare events” simulations. We describe here a method that consists in the simulation of short trajectories starting from an equilibrated transition state in solution, the structure of which has been approximately established. This calculation is particularly efficient when carried out with parallel computers since the study of a reactive process is decomposed in a set of short time trajectories that are completely independent. The procedure is close to that used by other authors in the context of classical molecular dynamics but present the advantage of describing the chemical system with rigorous quantum mechanical calculations. It is illustrated through the study of the first reaction step in electrophilic bromination of ethylene in water. This elementary process is representative of many charge separation reactions for which static and dynamic solvent effects play a fundamental role. tunon@uv.es

An efficient and highly vectorized implementation of the coupled cluster singles and doubles (CCSD) model using a direct atomic integral technique is presented. The minimal number of n6processes has been implemented for the most time consuming terms and point group symmetry is used to further reduce operation counts and memory requirements. The significantly increased application range of the CCSD method is illustrated with sample calculations on several systems with more than 500 basis functions. Furthermore, we present the basic trends of an open ended algorithm and discuss the use of integral prescreening. © 1996 American Institute of Physics.

A combined ab initio and experimental investigation has been performed of the main features of the electronic spectrum of acetone. Vertical transition energies have been calculated from the ground to the ny→π∗, π→π∗, σ→π∗, and the n=3 Rydberg states. In addition, the 1A1 energy surfaces have been studied as functions of the CO bond length. The 1A1 3p and 3d states were found to be heavily perturbed by the π→π∗ state. Resonant multiphoton ionization and polarization‐selected photoacoustic spectra of acetone have been measured and observed transitions were assigned on internal criteria. The calculated vertical transition energies to the ny→π∗ and all Rydberg states were found to be in agreement with experiment. This includes the 3s‐, all three 3p‐, and the A1, B1, and B2 3d‐Rydberg states. By contrast, there is little agreement between the calculated and experimental relative intensities of the A1 and B2 3d‐Rydberg transitions. In addition, anomalously intense high vibrational overtone bands of one of the 3p‐Rydberg transitions have been observed. These results confirm the strong perturbation of the 3p‐ and 3d‐Rydberg states by the π→π∗ state found in the theoretical calculation and support the calculated position of this unobserved state. merchan@uv.es

Intermediate Hamiltonians are effective Hamiltonians which are defined on an N‐dimensional model space but which only provide n

International audience; We present here the results of time-resolved absorption and emission experiments for 4-(dimethylamino)benzonitrile in solution, which suggest that the fluorescent intramolecular charge transfer (ICT) state may differ from the twisted ICT (TICT) state observed in transient absorption.

In this paper, we report angle-dispersive X-ray diffraction data of molybdenum melting, measured in a double-sided laser-heated diamond-anvil cell up to a pressure of 119 GPa and temperatures up to 3400 K. The new melting temperatures are in excellent agreement with earlier measurements up to 90 GPa that relied on optical observations of melting and in strong contrast to most theoretical estimates. The X-ray measurements show that the solid melts from the bcc structure throughout the reported pressure range and provide no evidence for a high temperature transition from bcc to a close-packed structure, or to any other crystalline structure. This observation contradicts earlier interpretations of shock data arguing for such a transition. Instead, the values for the Poisson ratios of shock compressed Mo, obtained from the sound speed measurements, and the present X-ray evidence of loss of long-range order suggest that the 210 GPa ( ∼ 4100 K) transition in the shock experiment is from the bcc structure to a new, highly viscous, structured melt. Daniel.Errandonea@uv.es

Some linear and nonlinear optical (NLO) properties of Ni(SCH)4 and several of its derivatives have been computed by employing a series of basis sets and a hierarchy of methods (e.g., HF, DFT, coupled cluster, and multiconfigurational techniques). The electronic structure of Ni(SCH)4 has been also analyzed by using CASSCF/CASPT2, ab initio valence bond, and DFT methods. In particular we discuss how the diradicaloid character (DC) of Ni(SCH)4 significantly affects its NLO properties. The quasidegeneracy of the two lowest-energy singlet states 1 mathg and 1 math1u, the clear DC nature of the former, and the very large number of low-lying states enhance the NLO properties values. These particular features are used to interpret the NLO properties of Ni(SCH)4. The DC of the considered derivatives has been estimated and correlated with the NLO properties. CASVB computations have shown that the structures with Ni(II) are the dominant ones, while those with Ni(0) and Ni(IV) have negligible weight. The weights of the four diradical structures were discussed in connection with the weight of the structures, where all the electrons are paired. Comparative discussion of the properties of Ni(SCH)4 with those of tetrathia fulvalene demonstrates the very large effect of Ni on the properties of the Ni-dithiolene derivatives. A similar remarkable effect on the NLO properties is produced by one or two methyl or C3S groups. The considered Ni-dithiolene derivatives have exceptionally large NLO properties. This feature in connection with their other physical properties makes them ideal candidates for photonic applications. Luis.Serrano@uv.es

It is predicted that oriented BeO molecules would give rise to unprecedentedly strong, unidirectional electric ring current and an associated magnetic field upon excitation by a right or left circularly polarized laser pulse into the first excited degenerate singlet state. The strong toroidal electric ring current of this state is dominated by the ring current of the 1π± orbital about the molecular axis. Our predictions are based on the analysis of the orbital composition of the states involved and are substantiated by high level electronic structure calculations and wavepacket simulations of the laser-driven orientation and excitation dynamics. Luis.Serrano@uv.es

An all-electron full configuration interaction (FCI) calculation of the adiabatic potential energy curves of some of the lower states of BeH molecule is presented. A moderately large ANO basis set of atomic natural orbitals (ANO) augmented with Rydberg functions has been used in order to describe the valence and Rydberg states and their interactions. The Rydberg set of ANOs has been placed on the Be at all bond distances. So, the basis set can be described as 4s3p2d1f3s2p1d(BeH)+4s4p2d(Be). The dipole moments of several states and transition dipole strengths from the ground state are also reported as a function of the R(Be-H) distance. The position and the number of states involved in several avoided crossings present in this system have been discussed. Spectroscopic parameters have been calculated from a number of the vibrational states that result from the adiabatic curves except for some states in which this would be completely nonsense, as it is the case for the very distorted curves of the 3s and 3p (2)Sigma(+) states or the double-well potential of the 4p (2)Pi state. The so-called "D complex" at 54 050 cm(-1) (185.0 nm) is resolved into the three 3d substates ((2)Sigma(+),(2)Pi,(2)Delta). A diexcited valence state is calculated as the lowest state of (2)Sigma(-) symmetry and its spectroscopic parameters are reported, as well as those of the 2 (2)Delta (4d) state The adiabatic curve of the 4 (2)Sigma(+) state shows a swallow well at large distances (around 4.1 A) as a result of an avoided crossing with the 3 (2)Sigma(+) state. The probability that some vibrational levels of this well could be populated is discussed within an approached Landau-Zerner model and is found to be high. No evidence is found of the E(4ssigma) (2)Sigma(+) state in the region of the "D complex". Instead, the spectroscopic properties obtained from the (4ssigma) 6 (2)Sigma(+) adiabatic curve of the present work seem to agree with those of the experimental F(4psigma) (2)Sigma(+) state. The FCI calculations provide benchmark results for other correlation models for the open-shell BeH system and evidence both the limitations and capabilities of the basis set.

Using Cholesky decomposition and density fitting to approximate the electron repulsion integrals, an implementation of the complete active space self-consistent field (CASSCF) method suitable for large-scale applications is presented. Sample calculations on benzene, diaquo-tetra- μ -acetato-dicopper(II), and diuraniumendofullerene demonstrate that the Cholesky and density fitting approximations allow larger basis sets and larger systems to be treated at the CASSCF level of theory with controllable accuracy. While strict error control is an inherent property of the Cholesky approximation, errors arising from the density fitting approach are managed by using a recently proposed class of auxiliary basis sets constructed from Cholesky decomposition of the atomic electron repulsion integrals. © 2008 American Institute of Physics.

We present a novel approach to the calculation of the Coulomb and exchange contributions to the total electronic energy in self consistent field and density functional theory. The numerical procedure is based on the Cholesky decomposition and involves decomposition of specific Hadamard product matrices that enter the energy expression. In this way, we determine an auxiliary basis and obtain a dramatic reduction in size as compared to the resolution of identity (RI) method. Although the auxiliary basis is determined from the energy expression, we have complete control of the errors in the gradient or Fock matrix. Another important advantage of this method specific Cholesky decomposition is that the exchange energy and Fock matrix can be evaluated with a linear scaling effort contrary to the RI method or standard Cholesky decomposition of the two-electron integral matrix. The methods presented show the same scaling properties as the so-called local density fitting methods, but with full error control. © 2008 American Institute of Physics.

We propose a perturbative criterion to select the most important dissociation‐ or correlation‐consistent type of contributions to perform generalized valence bond‐configuration interaction (GVB‐CI) calculations, dissociation‐consistent configuration interaction (DCCI) or correlation‐consistent configuration interaction (CCCI) approach, respectively. The procedure presented is computationally less demanding than the CCCI proposed by Goddard and co‐workers. To ensure the distance consistency of the MOs used, the nonvalence virtual orbitals are obtained by a projection technique. The results obtained for a few test calculations show the ability of the suggested approach to get close results to full CI, DCCI, and CCCI values using a small CI expansion. It seems to be a promising way to treat correlation changes in large molecular systems which would be inaccessible by other methods. Manuela.Merchan@uv.es ; Ignacio.Nebot@uv.es

We employ a series of state-of-the-art computational techniques to study the effect of inserting one or more Xe atoms in HC2H and HC4H, on the linear and nonlinear optical (L&NLO) properties of the resulting compounds. It has been found that the inserted Xe has a great effect on the L&NLO properties of the organoxenon derivatives. We analyze the bonding in HXeC2H, and the change of the electronic structure, which is induced by inserting Xe, in order to rationalize the observed extraordinary L&NLO properties. The derivatives, which are of interest in this work, have been synthesized in a Xe matrix. Thus the effect of the local field (LF), due to the Xe environment, on the properties of HXeC2H, has also been computed. It has been found that the LF effect on some properties is significant. The calculations have been performed by employing a hierarchy of basis sets and the techniques MP2 and CCSD(T) for taking into account correlation. For the interpretation of the results we have employed the complete active space valence bond and CASSCF/CASPT2 methods. Luis.Serrano@uv.es

Vertical excitation energies as well as related absolute photoabsorption oscillator strength data are very scarce in the literature for methane. In this study, we have characterized the three existing series of low-lying Rydberg states of CH4 by computing coupled cluster linear response (CCLR) vertical excitation energies together with oscillator strengths in the molecular-adapted quantum defect orbital formalism from a distorted Cs geometry selected on the basis of outer valence green function calculations. The present work provides a wide range of data of excitation energies and absolute oscillator strengths which correspond to the Rydberg series converging to the three lower ionization potential values of the distorted methane molecule, in energy regions for which experimentally measured data appear to be unavailable. Jose.V.Pitarch@uv.es Alfredo.Sanchez@uv.es Jose.Sanchez@uv.es

Quantum chemical high level ab initio coupled-cluster and multiconfigurational perturbation methods have been used to compute vertical and adiabatic ionization potentials of the five canonical DNA and RNA nucleobases: uracil, thymine, cytosine, adenine, and guanine. Several states of their cations have been also calculated. The present results represent a systematic compendium of these magnitudes, establishing theoretical reference values at a level not reported before, calibrating computational strategies, and guiding the assignment of the features in the experimental photoelectron spectra. Daniel.Roca@uv.es Mercedes.Rubio@uv.es Manuela.Merchan@uv.es Luis.Serrano@uv.es

Psoralen photophysics has been studied on quantum chemistry grounds using the multiconfigurational second-order perturbation method CASPT2. Absorption and emission spectra of the system have been rationalized by computing the energies and properties of the low-lying singlet and triplet excited states. The S1 ππ* state has been determined to be responsible of the lowest absorption and fluorescence bands and to initially carry the population in the photophysical processes related to the phototherapeutic properties of psoralen derivatives. The low-lying T1 ππ* state is, on the other hand, protagonist of the phosphorescence, and its prevalent role in the reactivity of psoralen is suggested to be related to the elongation of the pyrone ring C3–C4 bond, where the spin density is distributed on both carbon atoms. Analysis of energy gaps and spin-orbit coupling elements indicates that the efficient photophysical process leading to the population of the lowest triplet state does not take place at the Franck-Condon region but along the S1 relaxation path. Juan.Serrano@uv.es Luis.Serrano@uv.es Manuela.Merchan@uv.es

The ground-state energies and the radial and pair distribution functions of neutral math clusters are systematically calculated by the diffusion Monte Carlo method in steps of one math atom from 3 to 50 atoms. In addition the chemical potential and the low-lying excitation levels of each cluster are determined with high precision. These calculations reveal that the “magic numbers” observed in experimental math cluster size distributions, measured for free jet gas expansions by nondestructive matter-wave diffraction, are not caused by enhanced stabilities. Instead they are explained in terms of an enhanced growth due to sharp peaks in the equilibrium concentrations in the early part of the expansion. These peaks appear at cluster sizes which can just accommodate one more additional stable excitation. The good agreement with experiment provides not only experimental confirmation of the energy level and the chemical potential calculations, but also evidence for a new mechanism which can lead to magic numbers in cluster size distributions. By accounting for the falloff of the radial density distributions at the surface and a size-dependent surface tension, the energy levels are demonstrated to be consistent with a modified Rayleigh model of surface excitations. The compressibility coefficient of these small clusters is found to be one order of magnitude smaller than the bulk compressibility. Rafael.Guardiola@uv.es

Modern track-etching methods allow the preparation of membranes containing a single charged conical nanopore that shows high ionic permselectivity due to the electrical interactions of the surface pore charges with the mobile ions in the aqueous solution. The nanopore has potential applications in electrically assisted single-particle detection, analysis, and separation of biomolecules. We present a detailed theoretical and experimental account of the effects of pore radii and electrolyte concentration on the current-voltage and current-concentration curves. The physical model used is based on the Nernst-Planck and Poisson equations. Since the validity of continuum models for the description of ion transport under different voltages and concentrations is recognized as one of the main issues in the modeling of future applications, special attention is paid to the fundamental understanding of the electrical interactions between the nanopore fixed charges and the mobile charges confined in the reduced volume of the inside solution. Salvador.Mafe@uv.es

We present the results of an extended computational study of the electric and magnetic properties connected to Cotton-Mouton birefringences, on the trifluoro- and trichloroborides in the gas phase. The electric dipole polarizabilities, magnetizabilities, quadrupole moments, and higher-order hypersusceptibilities—expressed as quadratic and cubic frequency-dependent response functions—are computed within Hartree-Fock, density-functional, and coupled-cluster response theories employing singly and doubly augmented correlation-consistent basis sets and London orbitals in the magnetic property calculations. The results, which illustrate the capability of time-dependent density-functional theory for electron-rich systems, are compared with available experimental data. Revised values of both experimentally derived quadrupole moment of BF3, 2.72±0.15 a.u., and magnetizability anisotropy of BCl3, −0.45±0.09 a.u., both obtained in birefringence experiments that neglect the effects of higher-order hypersusceptibilities, are presented. In the theoretical limit the traceless quadrupole moments of BF3 and BCl3 are determined to be 3.00±0.01 and 0.71±0.01 a.u., respectively. Jose.M.Junquera@uv.es Alfredo.Sanchez@uv.es Jose.Sanchez@uv.es

Magnetic susceptibility and nuclear magnetic shielding at the nuclei of bis-heteropentalenes formed by two furan units ([2,3-b], [3,2-b], [3,4-b], and [3,4-c] isomers) have been computed by several approximated techniques and a large Gaussian basis set to achieve near Hartree–Fock estimates. Ab initio models of the ring currents induced by a magnetic field normal to the molecular plane were obtained for the three isomeric systems of higher symmetry, showing that the π electrons give rise to intense diamagnetic circulation. The π currents are responsible for enhanced magnetic anisotropy and strong out-of-plane proton deshielding. The theoretical findings are used to build up a “diatropicity matrix” for two fused five-membered heterocyclic systems. Inmaculada.Garcia-Cuesta@uv.es ; Alfredo.Sanchez@uv.es

The three-dimensional intermolecular electric dipole moment surface of Ar–CO is calculated at the coupled cluster singles and doubles level of theory with the aug-cc-pVTZ basis set extended with a 3s3p2d1f1g set of midbond functions. Using the rovibrational energies and wave functions of our recent study [J. Chem. Phys. 117, 6562 (2002)], temperature-dependent spectral intensities are evaluated and compared to available experimental data. Based on the theoretical spectrum, alternative assignments of the experimentally observed lines in the fundamental band of CO around 2160 and 2166 cm−1 are suggested. Thomas.Bondo@uv.es

The intermolecular potential-energy surface pertaining to the interaction between benzene and N2 is investigated theoretically and experimentally. Accurate intermolecular interaction energies are evaluated for the benzene–N2 van der Waals complex using the coupled cluster singles and doubles including connected triples [CCSD(T)] method and the aug-cc-pVDZ basis set extended with a set of 3s3p2d1f1g midbond functions. After fitting the energies to an analytic function, the intermolecular Schrödinger equation is solved to yield energies, rotational constants, and Raman-scattering coefficients for the lowest intermolecular levels of several benzene–N2 isotopomers. Experimentally, intermolecular Raman spectra of jet-cooled h6- and d6-benzene–N2 measured at 0.03 cm−1 resolution by mass-selective, ionization-detected stimulated Raman spectroscopies are reported. Seven intermolecular bands are assigned for each isotopomer, including transitions involving intermolecular bending and stretching vibrations and internal rotation about the benzene C6 axis. These Raman data, together with measured rotational constants and binding energies obtained by other groups on benzene–N2, agree well with the theoretical results. Such agreement points to the promise of the quantum chemical methodology employed herein in future investigations of larger van der Waals complexes. tbonpe@uv.es

The small numerical rank of the two-electron integral matrix for large molecular systems and large basis sets was demonstrated. Though, the current implementation still requires some improvements on the calculations done in the inner most loop of the decomposition do not exploit the parsity in the Cholesky vectors. With respect to the practical applicability of the presented method an efficient approach to geometrical derivatives was imperative. Such an approach was obtained including certain derivative product functions and decomposing an expanded integral matrix.

The vertical spectrum of tetracyanoethylene was studied using coupled cluster theory. It was found that the lowest singlet-singlet transition, which corresponds to the excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) excitation, occurs at 5.16 eV in the gas phase and is lowered approximately 0.1 eV due to solvent effects in acetonitrile. A parallel study on the ethene spectrum showed the quality of the basis sets and methods used, by placing the V state 7.92 eV above the ground state and giving an energy for the 0-0 transition of 5.42 eV to be compared with the experimental value of 5.50 eV.

A study of the intermolecular potential-energy surface (IPS) and the intermolecular states of the perprotonated and perdeuterated benzene–He complex is reported. From a fit to ab initio data computed within the coupled cluster singles and doubles including connected triples model for 280 interaction geometries, an analytic IPS including two- to four-body atom–atom terms is obtained. This IPS, and two other Lennard-Jones atom–atom surfaces from the literature, are each employed in dynamically exact (within the rigid-monomer approximation) calculations of J = 0 intermolecular states of the isotopomers. Rotational constants and Raman-scattering coefficients for intermolecular vibrational transitions are also calculated for each of the three surfaces. The calculated results are compared with experimental results reported herein pertaining to intermolecular Raman spectra of benzene–He. The calculated rotational constants are compared with experimental values from the literature. The fitted IPS of this work leads to calculated observables that match the experimental results very well. The IPSs from the literature are not as successful, specifically in regard to the intermolecular Raman spectra. Thomas.Bondo@uv.es

The dissociative electron transfer reaction CH3Cl+e−→CH3•+Cl− in aqueous solution is studied by using a QM/MM method. In this work the quantum subsystem (a methylchloride molecule plus an electron) is described using density functional theory while the solvent (300 water molecules) is described using the TIP3P classical potential. By means of molecular dynamics simulations and the thermodynamic integration technique we obtained the potential of mean force (PMF) for the carbon–chlorine bond dissociation of the neutral and radical anion species. Combining these two free energy curves we found a quadratic dependence of the activation free energy on the reaction free energy in agreement with Marcus’ relationship, originally developed for electron transfer processes not involving bond breaking. We also investigated dynamical aspects by means of 60 dissociative trajectories started with the addition of an extra electron to different configurations of a methylchloride molecule in solution. The PMF shows the existence of a very flat region, in which the system is trapped during some finite time if the quantum subsystem quickly losses its excess kinetic energy transferring it to the solvent molecules. One of the most important factors determining the effectiveness of this energy transfer seems to be the existence of close contacts (hydrogen bonds) between the solute and the solvent. Estanislao.Silla@uv.es Ignacio.Tunon@uv.es

The lowest-energy band of the electronic spectrum of pyrrole has been studied with vibrational resolution by using multiconfigurational second-order perturbation theory (CASPT2) and its multistate extension (MS–CASPT2) in conjunction with large atomic natural orbital-type basis sets including Rydberg functions. The obtained results provide a consistent picture of the recorded spectrum in the energy region 5.5–6.5 eV and confirm that the bulk of the intensity of the band arises from a ππ∗ intravalence transition, in contradiction to recent theoretical claims. Computed band origins for the 3s,3p Rydberg electronic transitions are in agreement with the available experimental data, although new assignments are suggested. As illustrated in the paper, the proper treatment of the valence–Rydberg mixing is particularly challenging for ab initio methodologies and can be seen as the main source of deviation among the recent theoretical results as regards the position of the low-lying valence excited states of pyrrole. serrano@uv.es ; merchan@uv.es

The first three-dimensional ab initio intermolecular potential energy surface of the Ar–CO van der Waals complex is calculated using the coupled cluster singles and doubles including connected triples model and the augmented correlation-consistent polarized valence quadruple zeta (aug-cc-pVQZ) basis set extended with a (3s3p2d1f1g) set of midbond functions. The three-dimensional surface is averaged over the three lowest vibrational states of CO. Rovibrational energies are calculated up to 50 cm−1 above the ground state, thus enabling comprehensive comparison between theory and available experimental data as well as providing detailed guidance for future spectroscopic investigations of higher-lying states. The experimental transitions are reproduced with a root-mean-square error of 0.13 cm−1, excluding states observed around 25 cm−1 above the ground state. The latter states are at variance with the experimentally deduced ordering.

We develop a physical model, based on the modified Donnan phenomena ideas introduced previously by the authors, to describe the acid doping of the conducting polymer polyaniline. The theory is motivated by the experimental work of Asturias et al. [Ber. Bunsenges. Phys. Chem. 95, 1381 (1991)]. Good agreement between theory and experiment is found.

The theory of the kinetic exchange in a pair of orbitally degenerate ions developed by the authors [J. Phys. Chem. A 102, 200 (1998)] is applied to the case of face-shared bioctahedral dimer (overall D3h-symmetry). The effective kinetic exchange Hamiltonian is found for a 2T2–2T2 system taking into account all relevant transfer pathways and charge-transfer crystal field states. The influence of different transfer integrals involved in the kinetic exchange on the energy pattern and magnetic properties of the system is examined. The role of other related interactions (trigonal crystal field, spin–orbit coupling) is also discussed in detail. Using the pseudoangular momentum representation and the technique of the irreducible tensor operators of R3-group we give a general outlook on the nontrivial symmetry properties of the effective Hamiltonian for the D3h-pair, and on the magnetic anisotropy arising from the orbital interactions specific for the case of orbital degeneracy. The magnetic properties of the binuclear unit [Ti2Cl9]−3 in Cs3Ti2Cl9 are discussed with a special emphasis on the magnetic anisotropy experimentally observed in this system. The existing exchange models for [Ti2Cl9]−3 and the concept of the effective Hamiltonian are discussed in the context of the present study. borrasj@uv.es ; clemente@uv.es ; coronado@uv.es ; palii@uv.es

We introduce an alternative to Almlöf and Häser’s Laplace transform decomposition of orbital energy denominators used in obtaining reduced scaling algorithms in perturbation theory based methods. The new decomposition is based on the Cholesky decomposition of positive semidefinite matrices. We show that orbital denominators have a particular short and size-intensive Cholesky decomposition. The main advantage in using the Cholesky decomposition, besides the shorter expansion, is the systematic improvement of the results without the penalties encountered in the Laplace transform decomposition when changing the number of integration points in order to control the convergence. Applications will focus on the coupled-cluster singles and doubles model including connected triples corrections [CCSD(T)], and several numerical examples are discussed. Alfredo.Sánchez@uv.es

The electronic excited states of p-benzoquinone have been studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbital (ANO) basis sets. The calculation of the singlet–singlet and singlet–triplet transition energies comprises 19 valence singlet excited states, 4 valence triplet states, and the singlet 3s,3p, and 3d members of the Rydberg series converging to the first four ionization limits. The computed vertical excitation energies are found to be in agreement with the available experimental data. Conclusive assignments to both valence and Rydberg states have been performed. The main features of the electronic spectrum correspond to the ππ∗ 1 1Ag→1 1B1u and ππ∗ 1 1Ag→3 1B1u transitions, computed to be at 5.15 and 7.08 eV, respectively. Assignments of the observed low-energy Rydberg bands have been proposed: An n→3p transition for the sharp absorption located at ca. 7.4 eV and two n→3d and π→3s transitions for the broad band observed at ca. 7.8 eV. The lowest triplet state is computed to be an nπ∗ 3B1g state, in agreement with the experimental evidence. pou@uv.es ; merchan@uv.es ; orti@uv.es

Glycine and alanine aminoacids chemistry in solution is explored using a hybrid three parameters density functional (B3PW91) together with a continuum model. Geometries, energies, and vibrational spectra of glycine and alanine zwitterions are studied at the B3PW91/6-31+G∗∗ level and the results compared with those obtained at the HF and MP2/6-31+G∗∗ levels. Solvents effects are incorporated by means of an ellipsoidal cavity model with a multipolar expansion (up to sixth order) of the solute’s electrostatic potential. Our results confirm the validity of the B3PW91 functional for studying aminoacid chemistry in solution. Taking into account the more favorable scaling behavior of density functional techniques with respect to correlated ab initio methods these studies could be extended to larger systems. silla@uv.es ; tunon@uv.es

A theoretical study of the vibrational structure of the math 2A′ ground and math 2A′ excited states of the formyl radical, HCO, and its deuterated form, DCO, has been performed. The potential energy surfaces have been computed by means of a multiconfigurational perturbative method, CASPT2. The computed geometries and the harmonic and anharmonic frequencies are successfully compared to the available experimental information. The vibrational intensities of the transition math 2A′↔math 2A′ have been computed both for absorption and emission. The results lead to accurate determinations of several structural parameters and some reassignments of the vibrational transitions of the so-called hydrocarbon flame bands of the formyl radical. serrano@uv.es

The self-consistent size consistent on a complete active space singly and doubly configuration interaction (SC)2CAS-SDCI method is applied to excited states. The (SC)2 correction is performed on a closed shell state, and the excited states are obtained by diagonalization of the dressed matrix. A theoretical justification of the transferability of the improvement concerning the dressing state to all roots of the matrix is presented. The method is tested by three tests on the spectrum of small molecules. sanchezm@uv.es ; nebot@uv.es

Ab initio results for the electronic spectrum of all-trans-retinal and its truncated model 3-methyl-all-trans (10-s-cis)-2,4,6,8,10-undecapentaen-1-al are presented. The study includes geometry determination of the ground state. Vertical excitation energies have been computed using multiconfigurational second-order perturbation theory through the CASPT2 formalism. The lowest singlet excited state in gas phase is predicted to be of nπ∗ character. The lowest triplet state corresponds, however, to a ππ∗ state. The most intense feature of the spectrum is due to the strongly dipole-allowed ππ∗ transition, in accordance with the observed maximum in the one-photon spectra. The vertical excitation energies of the Bu- and Ag-like states are found close, the latter ≈ 1 eV higher than the maximum in the two-photon spectra. Solvent effects and nonvertical nature of the observed maximum in the two-photon spectra are invoked in rationalizing the deviation with respect to the best present estimate for the Ag-like state. In addition, qualitative aspects of the one-bond photoisomerization about the C11 = C12 double bond of retinal are considered. The overall isomerization picture from 11-cis into all-trans-retinal, as taking place mainly along the triplet manifold, agrees with experimental evidence. merchan@uv.es ; gonzaler@uv.es

Limited previous experience with the mean value total dressing (MVTD) method had shown that MVTD energies for closed shell systems are generally better than CCSD(T) ones compared to FCI. The method, previously published as total dressing 2′(td-2′), is based on the single reference intermediate Hamiltonian theory. It is not a CC method but deals in a great part with the same physical effects that CC methods that incorporate amplitudes of triples such as CCSDT or its CCSDT-1n approaches. A number of test calculations comparing to diverse CC methods, as well as FCI and experiment when available, have been performed. The tests concern equilibrium energies in NH3 and CH2, equilibrium energies and distances in some diatomics (BF, NO+, CN+, C2, BeO), different bond breaking situations (H2O, BH, HF, SiH2) and spectroscopic properties of different bonding conditions (Li2, LiNa, LiBe+, NeH+, and O3). The results are in general closer to the full CCSDT ones in the equilibrium regions and close to CCSDT-1 along most dissociation curves. A few exceptions to this rule are analyzed with the help of an approach to MVTD that does not take into account the effects of linked quadriexcitations. Such analysis suggests the interest of improving the treatment of effects of linked triples in the MVTD model. The separate contributions of linked and unlinked triples and quadruples are also analyzed for some of the above diatomics representing different behaviors of bond breaking. The interest of such analysis is illustrated in the NeH+ molecule. The MVTD results show, in general, a high quality, provided that the nature of the correlation problem does not become largely multiconfigurational, as occurs in multiple bond dissociation or in the asymmetric stretching of ozone. garciain@uv.es ; sanchezm@uv.es ; sanchez@uv.es

An alternative derivation of many-body perturbation theory (MBPT) has been given, where a coupled cluster parametrization is used for the wave function and the method of undetermined Lagrange multipliers is applied to set up a variational coupled cluster energy expression. In this variational formulation, the nth-order amplitudes determine the energy to order 2n+1 and the nth-order multipliers determine the energy to order 2n+2. We have developed an iterative approximate coupled cluster singles, doubles, and triples model CC3, where the triples amplitudes are correct through second order and the singles amplitudes are treated without approximations due to the unique role of singles as approximate orbital relaxation parameters. The compact energy expressions obtained from the variational formulation exhibit in a simple way the relationship between CC3, CCSDT-1a [Lee et al., J. Chem. Phys. 81, 5906 (1984)] CCSDT-1b models [Urban et al., J. Chem. Phys. 83, 4041 (1985)], and the CCSD(T) model [Raghavachari et al., Chem. Phys. Lett. 157, 479 (1989)]. Sample calculations of total energies are presented for the molecules H2O, C2, CO, and C2H4. Comparisons are made with full CCSDT, CCSDT-1a, CCSDT-1b, CCSD(T), and full configuration interaction (FCI) results. These calculations demonstrate that CC3 and CCSD(T) give total energies of a similar quality. If results obtained by CC3 and CCSD(T) differ significantly, neither method can be trusted. In contrast to CCSD(T), time-dependent response functions can be obtained for CC3. © 1997 American Institute of Physics.

Algorithms for calculating singlet excitation energies in the coupled cluster singles and doubles (CCSD) model are discussed and an implementation of an atomic-integral direct algorithm is presented. Each excitation energy is calculated at a cost comparable to that of the CCSD ground-state energy. Singlet excitation energies are calculated for benzene using up to 432 basis functions. Basis-set effects of the order of 0.2 eV are observed when the basis is increased from augmented polarized valence double-zeta (aug-cc-pVDZ) to augmented polarized valence triple-zeta (aug-cc-pVTZ) quality. The correlation problem is examined by performing calculations in the hierarchy of coupled cluster models CCS, CC2, CCSD, and CC3, as well as by using the CCSDR(3) perturbative triples corrections. The effect of triple excitations are less than 0.2 eV for all excitations except for the 21E2gstate. The calculated excitation energies are compared with experiment and other theoretical results. © 1996 American Institute of Physics.

We report here a general solution of the double‐exchange problem in the high‐nuclearity mixed valence systems containing arbitrary number P of the electrons delocalized over the network of N (P

The role of counter ions and ion dissociation in establishing the equilibrium swelling of balanced and unbalanced polyampholytic hydrogels has been investigated experimentally and theoretically. The swelling dependence on both the net charge offset and the external bath salt concentration has been examined using an acrylamide based polyampholytic hydrogels. By careful consideration of the swelling kinetics, we illustrate the effects of ion dissociation equilibria and counter ion shielding in polyampholytic hydrogels near their balance point where both polyelectrolyte and polyampholyte effects are present. The theory considers a Flory type swelling model where the Coulombic interactions between fixed ions in the hydrogel resemble those of an ionic solid with a Debye screening factor. Theoretical predictions from this model are in qualitative agreement with our experimental results. manzanar@uv.es ; smafe@uv.es

The electronic spectrum of bithiophene in the energy range up to 6.0 eV has been studied using multiconfigurational second order perturbation theory (CASPT2) and a basis set of ANO type, with split valence quality and including polarization functions on all heavy atoms. Calculations were performed at a planar (trans) and twisted geometry. The calculated ordering of the excited singlet states is 1Bu, 1Bu, 1Ag, 1Ag, and 1Bu with 0–0 transition energies: 3.88, 4.15, 4.40, 4.71, and 5.53 eV, respectively. The first Rydberg transition (3s) has been found at 5.27 eV. The results have been used in aiding the interpretation of the experimental spectra, and in cases where a direct comparison is possible there is agreement between theory and experiment. rubio@uv.es ; merchan@uv.es ; orti@uv.es

This paper presents a new self‐consistent dressing of a singles and doubles configuration interaction matrix which insures size‐consistency, separability into closed‐shell subsystems if localized molecular orbitals (MOs) are used, and which includes all fourth order corrections. This method yields, among several schemes, a reformulation of the coupled cluster method, including fully the cluster operators of single and double excitations, and partially those of the triples (Bartlett’s algorithm named CCSDT‐1a). Further improvement can be easily included by adding exclusion principle violating corrections. Since it leads to a matrix diagonalization, the method behaves correctly in case of near degeneracies between the reference determinant and some doubles. Due to its flexibility this formulation offers the possibility of consistent combination with less expensive treatments for the study of very large systems. nebot@uv.es ; sanchezm@uv.es

We present the results of ab initio 3‐21G∗ geometry optimizations and valence effective Hamiltonian (VEH) band structure calculations aimed at determining the evolution of the geometric and electronic (ionization potential, electron affinities, and band gaps) properties of all‐trans poly(dimethylsilane), poly(diethylsilane), poly(di‐n‐propylsilane), and poly(di‐n‐butylsilane) when increasing the size of the alkyl group. In the latter polymer, we have also studied the 7/3 conformation, in order to analyze the effect of the backbone conformation on the geometric and electronic structure. The VEH ionization potentials of all‐trans poly(di‐n‐alkylsilanes) are almost equal, and as experimental photoemission data show, only slight differences are appreciated. The band gap decreases in going from poly(di‐n‐butylsilane) to poly(di‐n‐propylsilane) and to poly(diethylsilane), and increases when passing to poly(dimethylsilane), which coincides with experimental evidences on poly(di‐n‐alkylsilanes). The change from all‐trans to 7/3 conformation of poly(di‐n‐butylsilane) implies an increase of both, ionization potential and band gap, in perfect agreement with experimental photoemission and absorption data. The applicability of VEH to deal with poly(di‐n‐alkylsilanes) is discussed. crespo@uv.es