Approximate Spin Extended Hartree-Fock theory

An approximate spin-extended method is developed in which the Occupation Numbers of the Natural Orbitals of Charge are varied but the orbitals themselves are frozen. A number of techniques for obtaining appropriate Natural Orbitals of Charge are presented and these techniques are employed in semi-empirical and Ab-Initio studies. The molecules chosen for the Ab-Initio study, O₃, F₂ and C₂, are particularly interesting as Restricted Hartree-Fock (RHF) theory fails to provide an adequate description of these systems at their equilibrium geometries. The results of this work indicate that Approximate SpinExtended Hartree-Fock (ASEHF) theory provides a qualitatively acceptable description of bond dissociation, correctly predicts the symmetry of molecular states where RHF theory fails, and provides an efficient means for obtaining a significant amount of correlation energy in molecular calculations. Moreover, when the Generalized Valence Bond form of the ASEHF wavefunction is employed, a particularly useful description of the molecule is obtained. Specifically, the success or failure of the Woodward-Hoffman becomes transparent when the system is analyzed in terms of this wavefunction.