OPEN-SHELL RESTRICTED HARTREE-FOCK PERTURBATION-THEORY - SOME CONSIDERATIONS AND COMPARISONS

A comparative study is presented of the various recently developed ope n-shell perturbation theories that are based on a restricted Hartree-F ock reference wave function. Included in this study are issues concern ing spin contamination, implementational considerations, and numerical comparisons at the second-order of perturbation theory for equilibriu m geometries, vibrational frequencies, and singlet-triplet energy diff erences. Based on all of these considerations, it is concluded that th e z-averaged perturbation theory (ZAPT) method is to be preferred over the other recently devised spin-orbital perturbation theories, while the spin-free OPT2 method possesses some advantages and disadvantages relative to the ZAPT method. In particular, it is shown that OPT2 ener gies are not invariant to rotations among singly-occupied degenerate m olecular orbitals.