A NEW SPIN-RESTRICTED TRIPLE EXCITATION CORRECTION FOR COUPLED CLUSTER THEORY

摘要

A new approach to the noniterative inclusion of the effects of connected triple excitations in the coupled cluster singles and doubles (CCSD) model is presented. The method is based on the recently developed Z-averaged perturbation theory, and reduces to the usual Moller-Plesset-based triples correction in the absence of open shells. This new correction maintains the same invariance properties as those of the CCSD energy, and requires storage of only one-third the number of two-electron integrals as more conventional corrections. The derivation and implementation of the equations are described, and the method's performance relative to more conventional triples corrections is assessed via benchmark calculations for a series of diatomic molecules. (C) 1997 American Institute of Physics. [References: 52]