Restricted Møller—Plesset theory for open-shell molecules

Abstract

Møller—Plesset perturbation theory calculations for open-shell molecules based on a spin-restricted Hartree—Fock reference wavefunction are examined through the development of a new prescription for obtaining unique semi-canonical orbitals. These orbitals, which are different for α and β spins whilst avoiding the spin contamination present in UHF reference functions, satisfy the criteria on which Koopmans' theorem is based, lending justification to their use in defining a zeroth-order Hamiltonian for perturbation theory. This new and straightforward Møller—Plesset theory for open-shell molecules is called RMP theory. The convergence of the RMP series is examined to high order, and shows the greatly improved convergence characteristics also found with our alternative ROMP theory. For the molecules NH 2 ( r e)1.5 r e, 2 r e) and CN, RMP2 energies are substantially lower than UMP2 energies.