Desenvolvimento de metodos ab-initio para estudos envolvendo quebra de simetria em sistemas moleculares de grande porte




This doctoral thesis resumes the work performed by the present author in the period starting in August 1988 to January 1993. Three major points were studied during this job; a) Development and benchmarks for an original simetrization formula in ab-initio calculations; b) Modifications and extensive benchmarks in direct-SCF ab-initio calculations; c) Development of an original non-orthogonal orbital methodology based on symmetry reduced wave-functions coming from HF-SCF or MC-SCF calculations. The use of symmetry in ab-initio calculations is originated in the second half of seventies with the work of Dupuis and King. ln this thesis we revise and extend the use of the Clebsch-Gordan coupling coefficients, proposed by Pitzer, showing how this methodology is related with the simetrization formulas of Dupuis and King and Lazarretti et al in SCF calculations as well as in MP2 calculations. All this steps were implemented in commercial programs easily available. Another project investigated the effect of several methodological parameters in the efficiency of direct-SCF calculations. Among the principal conclusions we point out an improvement around 5% in the CPU time for calculations using the integrals organized under the form of super-matrices. The use of the SCF-partially direct was also studied with respect to the integral criteria storage and with respect to the efficiency in the parallelization of direct SCF calculations. With respect to the study of localized models we extended the work that Goddard and Voter had within the RES-HF and RES-GVB method towards the multi-structural configuration interaction, MEG-CI (RES-CI). We studied this type of wave-function applied to the three electron resonance molecular systems, ionization potentials and excited states of the trans-glioxal molecule. This work was complemented with CAS-SCF, GVB and MR-CI calculations performed on the trans-glioxal itself, pirazine and the trans-butadiene molecules. The use of localized models yields excellent results to the excitation energies of this systems.


simetria (quimica) orbitais moleculares

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