Aplicació de la semblança molecular quàntica en la reducció de l'espai configuracional per a l'estat fonamental i primers excitats de l'àtom d'Heli

Es presenta un mètode de selecció d'orbitals atòmics relacionat amb la teoria de la Semblança Molecular Quàntica, que permet reduir l'espai actiu quan es vol dur a terme un càlcul a nivell d'Interacció de Configuracions per a l'àtom d'heli

Electron localization function at the correlated level

The electron localization function (ELF) has been proven so far a valuable tool to determine the location of electron pairs. Because of that, the ELF has been widely used to understand the nature of the chemical bonding and to discuss the mechanism of chemical reactions. Up to now, most applications of the ELF have been performed with monodeterminantal methods and only few attempts to calculate this function for correlated wave functions have been carried out. Here, a formulation of ELF valid for mono- and multiconfigurational wave functions is given and compared with previous recently reported approaches. The method described does not require the use of the homogeneous electron gas to define the ELF, at variance with the ELF definition given by Becke. The effect of the electron correlation in the ELF, introduced by means of configuration interaction with singles and doubles calculations, is discussed in the light of the results derived from a set of atomic and molecular systems

The role of dynamical polarization of the ligand-to-metal charge transfer excitations in the ab initio determination of effective exchange parameters

The role of the bridging ligand on the effective Heisenberg coupling parameters is analyzed in detail. This analysis strongly suggests that the ligand-to-metal charge transfer excitations are responsible for a large part of the final value of the magnetic coupling constant. This permits us to suggest a variant of the difference dedicated configuration interaction (DDCI) method, presently one of the most accurate and reliable for the evaluation of magnetic effective interactions. This method treats the bridging ligand orbitals mediating the interaction at the same level than the magnetic orbitals and preserves the high quality of the DDCI results while being much less computationally demanding. The numerical accuracy of the new approach is illustrated on various systems with one or two magnetic electrons per magnetic center. The fact that accurate results can be obtained using a rather reduced configuration interaction space opens the possibility to study more complex systems with many magnetic centers and/or many electrons per center.

Modeling and manufacturability assessment of bistable quantum-dot cells

We have investigated the behavior of bistable cells made up of four quantum dots and occupied by two electrons, in the presence of realistic confinement potentials produced by depletion gates on top of a GaAs/AlGaAs heterostructure. Such a cell represents the basic building block for logic architectures based on the concept of quantum cellular automata (QCA) and of ground state computation, which have been proposed as an alternative to traditional transistor-based logic circuits. We have focused on the robustness of the operation of such cells with respect to asymmetries derived from fabrication tolerances. We have developed a two-dimensional model for the calculation of the electron density in a driven cell in response to the polarization state of a driver cell. Our method is based on the one-shot configuration-interaction technique, adapted from molecular chemistry. From the results of our simulations, we conclude that an implementation of QCA logic based on simple ¿hole arrays¿ is not feasible, because of the extreme sensitivity to fabrication tolerances. As an alternative, we propose cells defined by multiple gates, where geometrical asymmetries can be compensated for by adjusting the bias voltages. Even though not immediately applicable to the implementation of logic gates and not suitable for large scale integration, the proposed cell layout should allow an experimental demonstration of a chain of QCA cells.

Optimization of the valence energy variance of the CuH molecule

We developed the concept of split-'t to deal with the large molecules (in terms of the number of electrons and nuclear charge Z). This naturally leads to partitioning the local energy into components due to each electron shell. The minimization of the variation of the valence shell local energy is used to optimize a simple two parameter CuH wave function. Molecular properties (spectroscopic constants and the dipole moment) are calculated for the optimized and nearly optimized wave functions using the Variational Quantum Monte Carlo method. Our best results are comparable to those from the single and double configuration interaction (SDCI) method.

The role of dynamical polarization of the ligand-to-metal charge transfer excitations in the ab initio determination of effective exchange parameters

The role of the bridging ligand on the effective Heisenberg coupling parameters is analyzed in detail. This analysis strongly suggests that the ligand-to-metal charge transfer excitations are responsible for a large part of the final value of the magnetic coupling constant. This permits us to suggest a variant of the difference dedicated configuration interaction (DDCI) method, presently one of the most accurate and reliable for the evaluation of magnetic effective interactions. This method treats the bridging ligand orbitals mediating the interaction at the same level than the magnetic orbitals and preserves the high quality of the DDCI results while being much less computationally demanding. The numerical accuracy of the new approach is illustrated on various systems with one or two magnetic electrons per magnetic center. The fact that accurate results can be obtained using a rather reduced configuration interaction space opens the possibility to study more complex systems with many magnetic centers and/or many electrons per center.

Electron localization function at the correlated level

The electron localization function (ELF) has been proven so far a valuable tool to determine the location of electron pairs. Because of that, the ELF has been widely used to understand the nature of the chemical bonding and to discuss the mechanism of chemical reactions. Up to now, most applications of the ELF have been performed with monodeterminantal methods and only few attempts to calculate this function for correlated wave functions have been carried out. Here, a formulation of ELF valid for mono- and multiconfigurational wave functions is given and compared with previous recently reported approaches. The method described does not require the use of the homogeneous electron gas to define the ELF, at variance with the ELF definition given by Becke. The effect of the electron correlation in the ELF, introduced by means of configuration interaction with singles and doubles calculations, is discussed in the light of the results derived from a set of atomic and molecular systems

Aplicació de la semblança molecular quàntica en la reducció de l'espai configuracional per a l'estat fonamental i primers excitats de l'àtom d'Heli

Es presenta un mètode de selecció d'orbitals atòmics relacionat amb la teoria de la Semblança Molecular Quàntica, que permet reduir l'espai actiu quan es vol dur a terme un càlcul a nivell d'Interacció de Configuracions per a l'àtom d'heli

Vibrational energy levels for the electronic ground state of the diazocarbene (CNN) molecule

The vibrational energy levels of diazocarbene (diazomethylene) in its electronic ground state, (X) over tilde (3) Sigma(-) CNN, have been predicted using the variational method. The potential energy surfaces of (X) over tilde (3) A" CNN were determined by employing ab initio single reference coupled cluster with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], multi-reference complete active space self-consistent-field (CASSCF), and internally contracted multi-reference configuration interaction (ICMRCI) methods. The correlation-consistent polarised valence quadruple zeta (cc-pVQZ) basis set was used. Four sets of vibrational energy levels determined from the four distinct analytical potential functions have been compared with the experimental values from the laser-induced fluorescence measurements of Wurfel et al. obtained in 1992. The CCSD, CCSD(T), and CASSCF potentials have not provided satisfactory agreement with the experimental observations. In this light, the importance of both non-dynamic (static) and dynamic correlation effects in describing the ground state of CNN is emphasised. Our best theoretical fundamental frequencies at the cc-pVQZ ICMRCI level of theory, v(1) = 1230, v(2) = 394, and v(3) = 1420 cm(-1) are in excellent agreement with the experimental values of v(1) = 1235, v(2) = 396, and v(3) = 1419cm(-1) and the mean absolute deviation between the 23 calculated and experimental vibrational energy levels is only 7.4 cm(-1). It is shown that the previously suggested observation of the v(3) frequency at about 2847cm(-1) was in fact the first overtone 2v(3).

Ab initio prediction of the infrared-absorption spectrum of the C2Cl radical

The three lowest (1(2)A('), 2(2)A('), and 1(2)A(')) potential-energy surfaces of the C2Cl radical, correlating at linear geometries with (2)Sigma(+) and (2)Pi states, have been studied ab initio using a large basis set and multireference configuration-interaction techniques. The electronic ground state is confirmed to be bent with a very low barrier to linearity, due to the strong nonadiabatic electronic interactions taking place in this system. The rovibronic energy levels of the (CCCl)-C-12-C-12-Cl-35 isotopomer and the absolute absorption intensities at a temperature of 5 K have been calculated, to an upper limit of 2000 cm(-1), using diabatic potential-energy and dipole moment surfaces and a recently developed variational method. The resulting vibronic states arise from a strong mixture of all the three electronic components and their assignments are intrinsically ambiguous. (c) 2005 American Institute of Physics.

Ab initio prediction of the infrared absorption spectrum of the C2Br radical

The first three electronic states (1(2)A', 2(2)A', 1(2)A '') of the C2Br radical, correlating at linear geometries with (2)Sigma(+) and (2)Pi states, have been studied ab initio, using Multi Reference Configuration Interaction techniques. The electronic ground state is found to have a bent equilibrium geometry, R-CC = 1.2621 angstrom, R-CBr = 1.7967 angstrom, < CCBr 156.1 degrees, with a very low barrier to linearity. Similarly to the valence isoelectronic radicals C2F and C2Cl, this anomalous behaviour is attributed to a strong three-state non-adiabatic electronic interaction. The Sigma, Pi(1/2), Pi(3/2) vibronic energy levels and their absolute infrared absorption intensities at a temperature of 5K have been calculated for the (CCBr)-C-12-C-12-Br-79 isotopomer, to an upper limit of 2000 cm(-1), using ab initio diabatic potential energy and dipole moment surfaces and a recently developed variational method.

Structural and spectroscopic properties of the diazocarbene radical (CNN) and its ions CNN(+) and CNN(-): a high-level theoretical investigation

The diazocarbene radical, CNN, and the ions CNN(+) and CNN(-) were investigated at a high level of theory. Very accurate structural parameters for the states X(3)Sigma(-) and A(3)Pi of CNN, and X(2)Pi of both CNN(+) and CNN(-) were obtained with the UCCSD(T) method using correlated-consistent basis functions with extrapolations to the complete basis set limit, with valence only and also with all electrons correlated. Harmonic and anharmonic frequencies were obtained for all species and the Renner parameter and average frequencies evaluated for the Pi states. At the UCCSD(T)/CBS(T-5) level of theory, Delta(f)H(0 K) = 138.89 kcal/mol and Delta(f)H(298 K) = 139.65 kcal/mol were obtained for diazocarbene; for the ionization potential and the electron affinity of CNN, 10.969 eV (252.95 kcal/mol), and 1.743 eV (40.19 kcal/mol), respectively, are predicted. Geometry optimization was also carried out with the CASSCF/MRCI/CBS(T-5) approach for the states X(3)Sigma(-) A(3)Pi, and a(1)Delta of CNN, and with the CASSCF/MRSDCI/aug-cc-pVTZ approach for the states b(1)Sigma(+), c(1)Pi, d(1)Sigma(-), and B(3)Sigma(-), and excitation energies (T(e)) evaluated. Vertical energies were calculated for 15 electronic states, thus improving on the accuracy of the five transitions already described, and allowing for a reliable overview of a manifold of other states, which is expected to guide future spectroscopic experiments. This study corroborates the experimental assignment for the vertical transition X (3)Sigma(-) <- E (3)Pi.

Dye incorporation in polyphosphate gels: synthesis and theoretical calculations

ABSTRACT: In this work we described theoretical calculations on the electronic structure and optical properties of the dyes crystal violet and malachite green based in semiempirical methods (Parametric Method 3 and Intermediate Neglect of Differential Overlap / Spectroscopic - Configuration Interaction) and the synthesis of a new hybrid material based upon the incorporation of these dyes in an aluminum polyphosphate gel network. The samples are nearly transparent, free-standing thick films. The optical properties of the entrapped dyes are sensitive to chemical changes within the matrix caused either by gel aging or external stimulli such as exposition to acidic and basic vapors that can percolate within the matrix. Our theoretical modeling is in good agreement with the experimental results for the dyes.

Investigação das características I x V e C x V de NCPS puro, com nitrogênio substitucional carregado (-1 e +1) e com grupos doador (NO2)-aceitador (NH2) através de métodos derivados de hartree-fock

Neste trabalho, fizemos uma investigação sobre o estudo teórico das características I x V e C x V de Nanotubo Carbono de Parede Simples (NCPS) puro, com Nitrogênio substitucional carregado com cargas -1 (caracterizando um indicativo de dopagem tipo n) e +1 (caracterizando um indicativo de dopagem tipo p) e na presença de grupos doador (NO₂)-aceitador (NH₂), através da simulação computacional do estado fundamental de NCPS, bem como de sua estrutura eletrônica e propriedades ópticas, utilizando parametrizações semi-empíricas AM1 (Austin Mudel 1) e ZINDO/S-ClS (Zerner´s lntermediate Neglect of Differential Orbital/Spectroscopic - Cunfiguration lnteraction Single) derivadas da Teoria de Hartree-Fock baseada em técnicas de química quântica. Por meio deste modelo teórico analisamos as propriedades ópticas e eletrônicas, de maior interesse para esses materiais, a fim de se entender a melhor forma de interação desses materiais na fabricação de dispositivos eletrônicos, tais como TECs (Transistores de Efeito de Campo) ou em aplicações em optoeletrônica tais como DEL (Dispositivo Emissor de Luz). Observamos que NCPS com Nitrogênio substitucional apresentam defeitos conformacionais do tipo polarônico. Fizemos as curvas dos espectros UV-visível de Absorção para NCPS armchair e zigzag puro, com Nitrogênio substitucional carregado com cargas (-1 e +1) e na presença de grupos doador (NO₂)-aceitador (NH₂), quando perturbados por intensidades diferentes de campo elétrico. Verificamos que em NCPS zigzag ao aumentarmos a intensidade do campo elétrico, suas curvas sofrem grandes perturbações. Obtivemos as curvas p x E, I x V e C x V para esses NCPS, concluímos que NCPS armchair possui comportamento resistor, pois suas curvas são lineares e zigzag possui comportamento semelhante ao dos dispositivos eletrônicos importantes para o avanço tecnológico. Assim, nossos resultados estão de bom acordo com os resultados experimentais e teóricos de NCPS puro e com Nitrogênio encontrados na literatura.

Investigação da transição isolante-metal do CDM via algoritmos computacionais e estudo de nano-dispositivos orgânicos

Apresentamos neste trabalho um estudo teórico sobre polímeros orgânicos conjugados. É conhecido que estes sistemas, em geral semicondutores ou isolantes, sob dopagem química podem vir a adquirir propriedades elétricas de material condutor. E ainda, sob ação de campo elétrico, pequenos oligômeros podem apresentar comportamento equivalente ao de dispositivos usuais, mas com inúmeras vantagens como, por exemplo, tamanho extremamente reduzido (alguns nanômetros). Dessa forma no primeiro capítulo faremos uma breve introdução sobre polímeros orgânicos conjugados mostrando alguns resultados experimentais obtidos para o polímero 4-dicianometileno-4,4-ciclopenta [2,1-b: 3,4b’] ditiofeno – CDM, que é o objeto central de estudo desta dissertação. O capítulo 2 trata dos métodos quânticos utilizados. Citaremos a Teoria de Hartre-Fock (HF) e suas derivações semi-empíricas. A técnica de Interação de configuração (CI) e a Teoria do Funcional da Densidade (DFT) também serão tratadas neste capítulo. O capítulo 3 é dedicado a descrever as características de alguns dispositivos usuais como diodos e transistores. Aqui o fundamental é entender a composição, o funcionamento e principalmente, como se comportam suas curvas características corrente versus voltagem (IxV). Citaremos ainda alguns dispositivos eletrônicos extremamente pequenos. No capítulo 4 começa nossos resultados e discussões referentes a análise da transição isolante-metal em CDM sob ação de dopagem. Primeiramente a nível semiempírico, obtivemos a caracterização ótica de oligômeros de CDM neutro e na presença de defeitos conformacionais do tipo bipólarons negativo e positivo. Partindo de geometrias otimizadas via métodos AM1 e PM3 obtivemos o espectro de absorção para sistemas com e sem carga. A nível Hartree-Fock calculamos a Estrutura de Bandas e a Densidade de Estados (DOS) para o PCDM no estado neutro e dopado. O cálculo da DOS e da Dispersão foram realizados através de programas computacionais desenvolvidos aqui no Grupo de Física de Materiais da Amazônia (GFMA). Apresentamos ainda neste capítulo o espectro de absorção teórico para oligômeros de CDM com diversas configurações com geometrias totalmente otimizadas pelo DFT. No capítulo 5 temos os resultados relativos à análise de nanodispositivos baseados em tetrâmeros de CDM com e sem carga. As curvas do deslocamento de carga versus voltagem apresentam características de curvas de dispositivos usuais. Analisamos também o espectro de absorção teórico dos nanodispositivos para valores de tensão nula e em pontos de saturação de corrente nas regiões direta e reversa.