5TH-ORDER MOLLER-PLESSET PERTURBATION-THEORY - COMPARISON OF EXISTING CORRELATION METHODS AND IMPLEMENTATION OF NEW METHODS CORRECT TO 5TH-ORDER

Full fifth-order Møller-Plesset perturbation theory of electron correlation is presented in algebraic form and used to compare the behavior of other approximate methods that are size-consistent and exact for two electrons. Considering only single and double substitutions, quadratic configuration interaction (QCISD), coupled cluster (CCSD), and Brueckner doubles (BD) theories are shown to deviate from each other in fifth order. The BD method contains the most parts of the fifth-order energy in a correct manner. The corresponding methods with noniterative triples corrections QCISD(T), CCSD(T), and BD(T) are also analyzed. These methods are all correct in those parts of the fifth-order energy that are linear in the higher (triple, quadruple) substitutions. Finally, new noniterative corrections are proposed that lead to theories completely correct in fifth order. These are QCISD(TQ), CCSD(TQ), and BD(TQ). The first and third of these have been implemented and are compared with full configuration interaction results for some simple systems. © 1990 American Chemical Society.