Single-reference CCSD approach employing three- and four-body CAS SCF corrections: A preliminary study of a simple model

Abstract

The idea of correcting the single-reference coupled cluster (CC) method truncated at the pair cluster level (CCSD) by means of three- and four-body corrections coming from some external source [J. Paldus, J. Čížek, and M. Takahashi, Phys. Rev. A 30, 2193 (1984); J. Paldus and J. Planelles, Theor. Chim. Acta 89, 13 (1994)] is explored at the ab initio level using the CAS SCF wave functions as a source of the triply and quadruply excited cluster amplitudes. The method (referred to as CCSD-CAS) is applied to three simple systems based on minimum basis set and double-zeta models of the BH molecule, in which we continuously vary, respectively, (i) the two electron repulsion, (ii) the electronic charge, and (iii) the internuclear separation in order to explore the performance of the method in quasi-degenerate situations. Both the energies and the higher than pair cluster amplitudes are compared with the corresponding exact full configuration interaction (FCI) results. The relative importance of the three- and four-body cluster components is also examined. In all cases considered, the CCSD-CAS method provides the best result. © 1997 John Wiley & Sons, Inc.