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 cons idered. We focus on the CCD + ST(CCD) perturbative estimate of the sin gly and triply excited clusters due to Raghavachari [J. Chem. Phys. 82 4607 (1985)] and its ACPQ + ST(ACPQ) analog proposed by Paldus and Pi ecuch [Int. J. Quantum Chem. 42, 135 (1992)]. The latter approach comb ines the perturbative treatment of singles and triples with an approxi mate CCD theory corrected for connected quadruply excited clusters (AC PQ). We also consider the OSA version of the CCSD + T(CCSD) method (co upled-cluster [CC] approach with singly and doubly excited clusters an d noniterative perturbative account of triply excited clusters) introd uced by Urban et al. [J. Chem. Phys. 83, 4041 (1985)]. The explicit OS A 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 graphi cal methods of spin algebras. The CCD + ST(CCD), CCSD + T(CCSD), and A CPQ + ST(ACPQ) formalisms have been implemented and the general purpos e ab initio programs have been written using a newly developed procedu re for improving the convergence of the reduced linear equation method [P. Piecuch and L. Adamowicz, J. Chem. Phys. 100, 5857 (1994)]. Resul ts of the pilot calculations for few nondegenerate and quasi-degenerat e systems are presented and compared with the full configuration inter action data. (C) 1995 John Wiley and Sons, Inc.