The method of moments of coupled-cluster equations and the renormalized CCSD[T], CCSD(T), CCSD(TQ), and CCSDT(Q) approaches

This paper is the first in a series of papers on the new approach to the ma ny-electron correlation problem, termed the method of moments of coupled-cl uster equations (MMCC). A hierarchy of MMCC approximations, including the r enormalized and completely renormalized CCSD[T], CCSD(T), CCSD(TQ), and CCS DT(Q) methods, which can be viewed as generalizations of the well-known per turbative coupled-cluster CCSD[T], CCSD(T), CCSD(TQ(f)), and CCSDT(Q(f)) sc hemes, is introduced. In this initial study, an emphasis is placed on the a bility of the MMCC approach to describe bond breaking and large effects due to connected triples and quadruples by modifying the standard noniterative CC approaches, such as the popular CCSD(T) method. The performance of sele cted MMCC approaches, including the renormalized and completely renormalize d CCSD[T], CCSD(T), and CCSD(TQ) schemes, is illustrated by the results of pilot calculations for the HF and H2O molecules. (C) 2000 American Institut e of Physics. [S0021-9606(00)31623-3].