Coupled-cluster methods with internal and semi-internal triply and quadruply excited clusters: CCSDt and CCSDtq approaches

Extension of the closed-shell coupled-cluster (CC) theory to studies of bon d breaking and general quasidegenerate situations requires the inclusion of the connected triply and quadruply excited clusters, T-3 and T-4, respecti vely. Since the complete inclusion of these clusters is expensive, we explo re the possibility of incorporating dominant T-3 and T-4 contributions by l imiting them to active orbitals. We restrict T-3 and T-4 clusters to intern al or internal and semi-internal components using arguments originating fro m the multireference formalism. A hierarchy of approximations to standard C CSDT (CC singles, doubles, and triples! and CCSDTQ (CC singles, doubles, tr iples, and quadruples! schemes, designated as the CCSDt and CCSDtq approach es, is proposed and tested using the H2O and HF molecules at displaced nucl ear geometries and C-2 at the equilibrium geometry. It is demonstrated that the CCSDt and CCSDtq methods provide an excellent description of bond brea king and nondynamic correlation effects. Unlike perturbative CCSDT and CCSD TQ approaches, the CCSDt and CCSDtq approaches do not fail at large internu clear separations, in spite of using the restricted Hartree-Fock reference. All CCSDt and CCSDtq approaches are essentially n(6) procedures and yet th ey are shown to provide reliable information about T-3 and T-4 components, whose standard evaluation requires expensive n(8) and n(10) steps. (C) 1999 American Institute of Physics. [S0021-9606(99)30410-4].