DIFFERENT FORMS OF PERTURBATION-THEORY FOR THE CALCULATION OF THE CORRELATION-ENERGY

Different forms of perturbation theory for the calculation of correlation energy in both closed- and open-shell systems are discussed. For closed-shell systems, Epstein-Nesbet perturbation theory is compared with Moller-Plesset (MP) perturbation theory based on canonical Hartree-Fock orbitals and with MP theory based on internally consistent SCF orbitals. The traditional MP theory gives superior results despite its use of an inferior zeroth-order Hamiltonian. This behavior is rationalized in terms of the larger denominators present in the traditional MP theory. These conclusions are used to support the restricted open-shell perturbation methods proposed recently by Murray and Davidson, and these new methods are compared with spin-restricted Epstein-Nesbet theory and the unrestricted MP (UMP) approach.