Ab initioprojected-unrestricted Hartree-Fock calculations

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Abstract
This paper illustrates some applications of ab initio projected-unrestricted Hartree-Fock (PUHF) theory. The necessary modifications to earlier semiempirical applications of the theory are described, and two applications are presented. Calculation of nuclear hyperfine interactions for the methyl radical shows that the theory provides reasonable spin polarization in free radicals. Study of the ozone molecule provides interesting comparisons with recent generalized valence-bond and configuration-interaction studies. Exploiting the equivalence of spin and spatial projection operations when applied to many broken-symmetry UHF wavefunctions appreciably extends the scope of PUHF theory. This provides the possibility of obtaining open-shell descriptions of singlet molecular ground states. In particular, the lower energy states of ozone are predicted in the correct order.

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