Jie Song

Date of Award

1-3-2003

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Mark R Hoffman

Abstract

The concept of macroconfigurations in molecular electronic structure theory was developed. The approach was shown to be useful in building a much more compact reference space, in prescreening noninteracting configuration pairs, and in efficiently regenerating configurations. Using macroconfigurations, two multireference approaches to calculating dynamic electron correlation, Second-Order Generalized Van Vleck Perturbation Theory (GVVPT2) and Multireference Configuration Interaction Singles and Doubles with Perturbative Inclusion of Triple and Quadruple Excitations (MRCISD(TQ), have been realized. GVVPT2 has been shown to be a noniterative approximation to the theoretically solid Self-Consistent Quasi-Degenerate Perturbation Theory (SC-QDPT). GVVPT2 was proven to be a useful alternative to the accurate MRCISD approach with substantially less computational requirements. The intruder state problem was avoided, and GVVPT2 is free of any singularities or discontinuities on potential energy surfaces. Representative calculations on model systems showed the wide applicability of this formalism. MRCISD(TQ) essentially required active use of macroconfigurations, since all known realizations of integral-driven CI programs are restricted to single and double excitations. A noniterative TQ energy correction was added to MRCISD, and the upper and lower bounds of the exact solution were shown to be estimable in an efficient way. From results of a few test cases, it seems that MRCISD(TQ is a viable candidate for ultrahigh accuracy (i.e. post-MRCISD) calculations. Theoretical investigations were carried on difluorodioxiranes and related molecules. The unimolecular rearrangement from CF2O2 to FC(=O)OF was studied at the MP2/cc-pVDZ level. Results showed it is a multi-step process with a high potential barrier (80 kcal/mol). As a result, a catalyst is required. Six excited states were investigated using the newly developed GVVPT2 code with cc-pVDZ basis set and their geometries and excitation energies were predicted. Four of them are determined to be low-lying and should be possible to observe experimentally. Fluorofluoroxy dioxirane (FC(OF)O2), an analog of difluorodioxirane, was investigated at the MP2/cc-pVDZ level. Six new isomers were identified, and their molecular properties were predicted. The global minimum was the molozonide with a CO3 ring. The unimolecular isomerization was studied among these isomers, and it was shown to be a complex process with high-energy intermediates and transition states involved.

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