Abstract
A new approach to the many-electron correlation problem, termed the method of moments of coupled-cluster equations (MMCC), is further developed and tested. The main idea of the MMCC theory is that of the noniterative energy corrections which, when added to the energies obtained in the standard coupled-cluster calculations, recover the exact (full configuration interaction) energy. The MMCC approximations require that a guess is provided for the electronic wave function of interest. The idea of using simple estimates of the wave function, provided by the inexpensive configuration interaction (CI) methods employing small sets of active orbitals to define higher–than–double excitations, is tested in this work. The CI-corrected MMCC methods are used to study the single bond breaking in HF and the simultaneous breaking of both O–H bonds in H2O.
Highlights
One of the most important problems in coupled-cluster (CC) theory [1,2,3,4,5,6,7] is extension of the standard single-reference CC (SRCC) method to quasi-degenerate electronic states
We demonstrated that the moments of coupledcluster equations (MMCC)(2,3) and MMCC(2,4) approximations, in which very simple noniterative corrections due to triples [the MMCC(2,3) case] or triples and quadruples [the MMCC(2,4) case] are added to the CCSD energy, provide excellent description of the single bond breaking in HF and the simultaneous breaking of both O–H bonds in H2O
The configuration interaction (CI)-based MMCC results obtained in this study are comparable to the highly accurate results for HF and H2O obtained with the many-body perturbation theory (MBPT)-based MMCC(2,3) and MMCC(2,4) approximations, referred to as the CR-CCSD(T) and CR-CCSD(TQ) approaches [7, 16,17,18, 22]
Summary
One of the most important problems in coupled-cluster (CC) theory [1,2,3,4,5,6,7] is extension of the standard single-reference CC (SRCC) method to quasi-degenerate electronic states. The standard SRCC methods, including the popular CCSD (CC singles and doubles) approach [8] and its CCSD[T] [9, 10], CCSD(T) [11], and CCSD(TQf) [12] extensions, in which the effects due to triply or triply and quadruply excited clusters are estimated using the arguments originating from the many-body perturbation theory (MBPT), fail to describe bond dissociation We focus on the single-reference MMCC approach, which, according to several preliminary studies [7, 16,17,18,19,20,21,22], allows us to accurately calculate molecular PESs involving bond breaking, while retaining the simplicity and ease-of-use of the popular noniterative SRCC approximations, such as CCSD(T). The CI-corrected MMCC approaches discussed in this work are tested in pilot calculations for the potential energy curve of HF and simultaneous breaking of both O–H bonds in H2O
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