Multireference perturbation configuration interaction. V. Third-order energy contributions in the Moller-Plesset and Epstein-Nesbet partitions.

A simple implementation of third-order perturbation theory applied to a multireference zero-order wavefunction is presented. Two different partitions of the Hamiltonian (Møller-Plesset baricentric and Epstein-Nesbet) are considered. Two test cases, CH2 and N2, are examined. The third-order results are shown to be in good agreement in either partition and are generally an improvement with respect to the second-order results. The phenomenon of intruder states, absent in Epstein-Nesbet, appears to be magnified in the Møller-Plesset partition.