Moeller-Plesset perturbation theory with spin projection

Abstract

A method has been developed for calculating energies by full spin projection of unrestricted Mdler-Plesset perturbation theory wave functions. The spin projection technique has been tested on bond dissociation potentials of LiH and HF and on symmetrically stretched H20. In the region where the UHF wave function is more stable than the RHF wave function, spin projected UMPn energies of low order (n I 4) have smaller errors than the corresponding spin restricted MPn and unprojected UMPn energies, when compared to full configuration interaction calculations. For higher order perturbation theory, spin restricted MPn energies may be more accurate than spin projected UMPn, but only for limited region near the RHF/UHF instability. An approximate spin projected UMPn formalism developed earlier yields energies that are in good agreement with the-present full spin projected UMPn calculations. A formula for spin projected energies in the coupled clusters approach is also presented. It is shown that annihilation of any single spin contaminant leaves the CCSD energy una1 tered.