ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
Extension of the closed-shell coupled-cluster (CC) theory to studies of bond breaking and general quasidegenerate situations requires the inclusion of the connected triply and quadruply excited clusters, T3 and T4, respectively. Since the complete inclusion of these clusters is expensive, we explore the possibility of incorporating dominant T3 and T4 contributions by limiting them to active orbitals. We restrict T3 and T4 clusters to internal or internal and semi-internal components using arguments originating from the multireference formalism. A hierarchy of approximations to standard CCSDT (CC singles, doubles, and triples) and CCSDTQ (CC singles, doubles, triples, and quadruples) schemes, designated as the CCSDt and CCSDtq approaches, is proposed and tested using the H2O and HF molecules at displaced nuclear geometries and C2 at the equilibrium geometry. It is demonstrated that the CCSDt and CCSDtq methods provide an excellent description of bond breaking and nondynamic correlation effects. Unlike perturbative CCSDT and CCSDTQ approaches, the CCSDt and CCSDtq approaches do not fail at large internuclear separations, in spite of using the restricted Hartree–Fock reference. All CCSDt and CCSDtq approaches are essentially n6 procedures and yet they are shown to provide reliable information about T3 and T4 components, whose standard evaluation requires expensive n8 and n10 steps. © 1999 American Institute of Physics.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.478517
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