COUPLED-CLUSTER APPROACHES WITH AN APPROXIMATE ACCOUNT OF TRIPLY AND QUADRUPLY EXCITED CLUSTERS - IMPLEMENTATION OF THE ORTHOGONALLY SPIN-ADAPTED CCD PLUS ST(CCD), CCSD PLUS T(CCSD), AND ACPQ PLUS ST(ACPQ) FORMALISMS

The orthogonally spin-adapted (OSA) coupled-pair (CCD) methods with an approximate account of triply and quadruply excited clusters are considered. We focus on the CCD + ST(CCD) perturbative estimate of the singly and triply excited clusters due to Raghavachari [J. Chem. Phys. 82 4607 (1985)] and its ACPQ + ST(ACPQ) analog proposed by Paldus and Piecuch [Int. J. Quantum Chem. 42, 135 (1992)]. The latter approach combines the perturbative treatment of singles and triples with an approximate CCD theory corrected for connected quadruply excited clusters (ACPQ). We also consider the OSA version of the CCSD + T(CCSD) method (coupled-cluster [CC] approach with singly and doubly excited clusters and noniterative perturbative account of triply excited clusters) introduced by Urban et al. [J. Chem. Phys. 83, 4041 (1985)]. The explicit OSA expressions for the previously neglected [P. Piecuch and J. Paldus, Theor. Chim. Acta 78, 65 (1990)] S(CCD) and S(ACPQ) terms are derived using diagrammatic methods of many-body perturbation theory and graphical methods of spin algebras. The CCD + ST(CCD), CCSD + T(CCSD), and ACPQ + ST(ACPQ) formalisms have been implemented and the general purpose ab initio programs have been written using a newly developed procedure for improving the convergence of the reduced linear equation method [P. Piecuch and L. Adamowicz, J. Chem. Phys. 100, 5857 (1994)]. Results of the pilot calculations for few nondegenerate and quasi-degenerate systems are presented and compared with the full configuration interaction data. (C) 1995 John Wiley and Sons, Inc.