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Quantum chemical calculations on hf and some related molecules Bruce, Robert Emerson

Abstract

This thesis reports some quantum chemical calculations directed at elucidating principles useful for refining calculations of electron distribution and other properties for complex molecules. In this work calculations have been made with the valence bond and molecular orbital methods using minimum basis sets of Slater-type orbitals on the ground states of the molecules HF and HO, and on states of HF⁺ corresponding to the ionization of either a 1s electron or a 2pπ electron from fluorine in HF. Calculations have been made for molecular energies, bond lengths, force constants, dipole moments, and electron distributions as given by Mulliken population analysis. For HF, the perfect pairing model with molecule-optimized exponents yields molecular energies about 6 kcal./mole lower than the comparable molecular orbital calculations; the dipole moment calculated by the perfect pairing method is 0.3 D. closer to the experimental value (1.82 D.) than that calculated by the molecular orbital method. The HF equilibrium bond length and force constants are calculated to a reasonable degree of accuracy with the two methods, although the first ionization potentials seem to be better calculated by the molecular orbital method either by Koopman's Theorem or by taking the difference between the energies of the two states. The calculations reported in this thesis show clearly that in general free atom exponents are not reliable for calculating molecular properties, and this is important for calculations on larger molecules which most frequently use basis functions appropriate to free atoms. As part of a programme for finding ways of optimizing exponents relatively inexpensively, for use with more complex molecules, an approximation due to Lowdin, for overlap charge distributions in electron repulsion integrals, was tested. The results reported in this thesis show that the method has promise in providing a way of initially optimizing exponents prior to the actual calculation wherein all integrals are evaluated exactly.

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