Ab initio molecular orbital theory has been successful in predicting the stabilities of many weak complexes; typical of these are the complexes formed between water and various small molecules. To account for the correlation effect, Moller-Plesset perturbation theory truncated at the second order level was employed. In order to account for the hydrogen bonding, the 6-3lG** basis set was used. The geometry optimisations of the complexes were carried out using the Gaussian-92 suite of programs installed on a Hewlett-Packard 720 computer operating under UNIX. The interaction energies of the complexes were subjected to further analysis by applying the Morokuma decomposition scheme. The electrostatic interaction component accounts for over 40% of the total stabilisation energy in all the typical hydrogen bonded complexes. Gas phase enthalpies were computed and compared with the experimental values of similar systems. For the systems studied here, the prediction is that all complexes are stable at 25° C. A second program, Vibra, was used for carrying out a normal coordinate analysis. A third computer program for the graphical representation of molecular and crystallographic models, Schakal-92, was employed to illustrate the predicted equilibrium geometries and the fundamental vibrational modes. The predicted geometries, interaction energies, charge redistributions, vibrational wave numbers, infrared intensities and force constants are listed and compared with those in the literature, where applicable. Correlations between the various predicted properties show some interesting chemistry. / Thesis (Ph.D.)-University of Natal, Durban, 1996.

Conjuntos de funções de base Gaussianos são desenvolvidos para os átomos da primeira fila dos metais de transição Sc - Zn. Esses conjuntos de base foram construídos por meio do método da Coordenada Geradora Hartree-Fock (GCHF - Generator Coordinate Hartre-Fock) baseado em uma expansão polinomial de grau 3 para discretizar as equações Griffin-Wheeler-Hartree-Fock. Neste procedimento, a maneira na qual as equações são discretizadas está baseada em uma malha de pontos flexíveis não igualmente espaçada para cada uma das simetrias orbitais requeridas para descrever os átomos estudados, a diferença do método GCHF original é que a malha é igualmente espaçada para todas as simetrias. Inicialmente, foi gerado um conjunto de base balanceado consistindo de 23s17p13d funções gaussianas primitivas. A partir deste, um conjunto padrão de qualidade 7Z na valência foi construído e, posteriormente, enriquecido com grupos de funções de polarização e gerando assim:pGCHF - 7Z - 2f 1g, pGCHF - 7Z - 3f 2g e pGCHF - 7Z - 3f 2g1h. Energias atômicas Hartree-Fock para os dois estados eletrônicos de menor energia, para os átomos do Sc - Zn, foram calculadas com nossos conjuntos e comparadas com valores da energia numérica. Os resultados apresentaram um erro máximo de 1.02 mHartree, demostrando a capacidade desta expansão polinomial no desenvolvimento de conjuntos de base acurados para átomos dos metais de transição (MT) 3d. Cálculos em nível da Teoria do Funcional da Densidade (DFT - Density Functional Theory), utilizando nove diferentes funcionais, foram realizados com nossos conjuntos de base. A energia eletrônica total e propriedades incluíndo: geometrias otimizadas, cumprimentos de ligação e, frequências vibracionais, foram examinadas para um conjunto de sistemas moleculares (hidretos, dicloretos, dímeros, trímeros e óxidos de metais de transição). Os resultados são comparados com valores teóricos obtidos com conjuntos de base cc-pVnZ (n= Q ou 5) e com valores experimentais, quando disponiveís na literatura. Os resultados mostram que os valores de energia total cc-pV5Z, podem ser atingidos com nossos conjuntos de base com um menor número de funções de polarização. Outros cálculos moleculares dão resultados aproximados com valores experimentais e valores de referência DFT/cc-pV5Z. O ponto mais importante para ser mencionado é que os conjuntos de base da coordenada geradora requerem somente uma pequena fração de tempo computacional para alcançar a convergência, quando comparados com cálculos DFT/cc-pVQZ e DFT/cc-pV5Z. / Gaussian basis set functions have been constructed for the atoms of the first row of the transition metals from Sc to Zn. These basis sets were built by means of the Generator Coordinate Hartree-Fock (GCHF) method based on a polynomial expansion of degree 3 for the discretization of the Grifin-Wheler-Hartree-Fock equations. In this procedure, the equations were discretized through a mesh which is not equally spaced for each one of the orbital symmetries required to describe the atoms studied in this work, differently from the original method, in which the mesh is equally spaced for all symmetries. At first, it was generated a minimal basis set consisting of 23s17p13d primitive functions. Starting with this pattern set was construted a set of 7Z quality in the valence and then, this one was enriched with polarization functions classified as: 2f1g, 3f2g y 3f2g1h, which originated 3 sets of basis functions namely: pGCHF-7Z-2f1g, pGCHF-7Z-3f2g y pGCHF-7Z-3f2g1h. Hartree-Fock energies for the atoms Sc-Zn were calculated with our basis sets, for the two electronic states with the lowest energy and compared with values of numerical energy. The maximum error presented for the results was 1.02mH, showing the ability of this polynomial expansion to create accurate basis sets for atoms of third-rows transition metals. Density Functional Theory (DFT) calculations using a set of nine functionals were realized with our basis sets. The total electronic energy and properties such as: optimized geometries, bond distances and vibrational frequencies were calculated for a set of molecules (hydrides, dichlorides, dimers, trimmers and oxides). The obtained values were compared with theoretical values obtained with calculations using the basis set cc-pVnZ and with some experimental values found in the literature. The outcomes showed that reference values of the total electronic can be reproduced by ours basis sets, although ours have a less degree of polarization. Other molecular calculations yield results very close to experimental values and reference theorical values calculated with DFT/cc-pV5Z. The most important point to be mentioned here is that our generator coordinate basis sets require only a tiny fraction of the computational time when compared to DFT/cc-pV5Z calculations.

O Método da Coordenada Geradora Hartree-Fock Polinomial (pMCG-HF), desenvolvido por R.C. Barbosa e A.B.F. da Silva [1], é uma ferramenta matemática valiosa que permite gerar funções de base (também conhecidas como conjuntos de base). As funções de base geradas por este método têm um bom comportamento e são capazes de calcular valores precisos de propriedades eletrônicas moleculares. Porém, depois de gerar funções de base do hidrogênio até o flúor [2], fez-se necessário a adição de expoentes à função de base, correspondentes a cada átomo, para melhor adaptação à realização dos cálculos moleculares. Estas funções adicionais são o que chamamos de funções de polarização. A adição de funções de polarização, através de otimização computacional, é muito custosa, deste modo o desenvolvimento de uma regra de polarização para se esquivar desta otimização é de grande importância e por isso se transforma na beleza e no objetivo deste trabalho. Portanto, nesta dissertação, estudar-se-á um procedimento para escolher funções de polarização que reduza drasticamente o tempo computacional, no sentido de permitir uma seleção, mais simples, de expoentes da própria função de base primitiva para serem usadas nas funções de polarização p, d, f, g, etc. para a obtenção de propriedades moleculares calculadas através de métodos químico-quânticos / The polynomial generate coordinate method pGCM developed by R.C. Barbosa and A.B.F. da Silva [1] is an remarkble mathematic tool for the generation of basis functions (also known as basis sets). The basis sets generated from this method have a good behavior and are able to produce accurate values for electronic molecular properties. In fact, after generating a basis set [2] we need to add a set of exponent functions in order to better adequate a basis set to perform molecular calculations. These sets of additional functions are called polarizations functions. This work provides a methodology where the polarization functions are obtained from the initial basis set (the primitive set) without optimizing them separately by using optimization algorithms that are, computationally speaking, very costly. This procedure reduces drastically the computational time used to find polarization functions to be used in molecular quantum chemical calculations. Our methodology permits to choose the polarization functions directly from the primitive orbital exponents of each atomic symmetry s, p, d, f etc. in a very simple manner. The finding of polarization functions using our methodology was performed with several quantum chemical methods.

O Método da Coordenada Geradora Hartree-Fock Polinomial (pMCG-HF), desenvolvido por R.C. Barbosa e A.B.F. da Silva [1], é uma ferramenta matemática valiosa que permite gerar funções de base (também conhecidas como conjuntos de base). As funções de base geradas por este método têm um bom comportamento e são capazes de calcular valores precisos de propriedades eletrônicas moleculares. Porém, depois de gerar funções de base do hidrogênio até o flúor [2], fez-se necessário a adição de expoentes à função de base, correspondentes a cada átomo, para melhor adaptação à realização dos cálculos moleculares. Estas funções adicionais são o que chamamos de funções de polarização. A adição de funções de polarização, através de otimização computacional, é muito custosa, deste modo o desenvolvimento de uma regra de polarização para se esquivar desta otimização é de grande importância e por isso se transforma na beleza e no objetivo deste trabalho. Portanto, nesta dissertação, estudar-se-á um procedimento para escolher funções de polarização que reduza drasticamente o tempo computacional, no sentido de permitir uma seleção, mais simples, de expoentes da própria função de base primitiva para serem usadas nas funções de polarização p, d, f, g, etc. para a obtenção de propriedades moleculares calculadas através de métodos químico-quânticos / The polynomial generate coordinate method pGCM developed by R.C. Barbosa and A.B.F. da Silva [1] is an remarkble mathematic tool for the generation of basis functions (also known as basis sets). The basis sets generated from this method have a good behavior and are able to produce accurate values for electronic molecular properties. In fact, after generating a basis set [2] we need to add a set of exponent functions in order to better adequate a basis set to perform molecular calculations. These sets of additional functions are called polarizations functions. This work provides a methodology where the polarization functions are obtained from the initial basis set (the primitive set) without optimizing them separately by using optimization algorithms that are, computationally speaking, very costly. This procedure reduces drastically the computational time used to find polarization functions to be used in molecular quantum chemical calculations. Our methodology permits to choose the polarization functions directly from the primitive orbital exponents of each atomic symmetry s, p, d, f etc. in a very simple manner. The finding of polarization functions using our methodology was performed with several quantum chemical methods.

The electronic structure of the zero-gap two-dimensional graphene has a charge neutrality point exactly at the Fermi level that limits the practical application of this material. There are several ways to modify the Fermi-level-region of graphene, e.g. adsorption of graphene on different substrates or different molecules on its surface. In all cases the so-called dispersion or van der Waals interactions can play a crucial role in the mechanism, which describes the modification of electronic structure of graphene. The adsorption of water on graphene is not very accurately reproduced in the standard density functional theory (DFT) calculations and highly-accurate quantum-chemical treatments are required. A possibility to apply wavefunction-based methods to extended systems is the use of local correlation schemes. The adsorption energies obtained in the present work by means of CCSD(T) are much higher in magnitude than the values calculated with standard DFT functional although they agree that physisorption is observed. The obtained results are compared with the values available in the literature for binding of water on the graphene-like substrates. / Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

The development of the correlation consistent basis sets, cc-pVnZ (where n = D, T, Q, etc.) have allowed for the systematic elucidation of the intrinsic accuracy of ab initio quantum chemical methods. In density functional theory (DFT), where the cc-pVnZ basis sets are not necessarily optimal in their current form, the elucidation of the intrinsic accuracy of DFT methods cannot always be accomplished. This dissertation outlines investigations into the basis set requirements for DFT and how the intrinsic accuracy of DFT methods may be determined with a prescription involving recontraction of the cc-pVnZ basis sets for specific density functionals. Next, the development and benchmarks of a set of cc-pVnZ basis sets designed for the s-block atoms lithium, beryllium, sodium, and magnesium are presented. Computed atomic and molecular properties agree well with reliable experimental data, demonstrating the accuracy of these new s-block basis sets. In addition to the development of cc-pVnZ basis sets, the development of a new, efficient formulism of the correlation consistent Composite Approach (ccCA) using the resolution of the identity (RI) approximation is employed. The new formulism, denoted 'RI-ccCA,' has marked efficiency in terms of computational time and storage, compared with the ccCA formulism, without the introduction of significant error. Finally, this dissertation reports three separate investigations of the properties of FOOF-like, germanium arsenide, and silicon hydride/halide molecules using high accuracy ab initio methods and the cc-pVnZ basis sets.

The electronic structure of the zero-gap two-dimensional graphene has a charge neutrality point exactly at the Fermi level that limits the practical application of this material. There are several ways to modify the Fermi-level-region of graphene, e.g. adsorption of graphene on different substrates or different molecules on its surface. In all cases the so-called dispersion or van der Waals interactions can play a crucial role in the mechanism, which describes the modification of electronic structure of graphene. The adsorption of water on graphene is not very accurately reproduced in the standard density functional theory (DFT) calculations and highly-accurate quantum-chemical treatments are required. A possibility to apply wavefunction-based methods to extended systems is the use of local correlation schemes. The adsorption energies obtained in the present work by means of CCSD(T) are much higher in magnitude than the values calculated with standard DFT functional although they agree that physisorption is observed. The obtained results are compared with the values available in the literature for binding of water on the graphene-like substrates. / Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.