Multireference coupled-cluster method using a single-reference formalism

Abstract

A multireference coupled-cluster singles and doubles method utilizing two reference determinants which differ by a two electron excitation is proposed. One of these determinants is selected as the formal reference determinant. The proposed method includes single-reference coupled-cluster equations truncated after quadruples. These equations are graphically derived using Feynman diagrams. The appropriate restrictions are then placed on the triple and quadruple amplitudes to allow only those amplitudes which correspond to single and double excitations from the second reference determinant.

Multi-reference coupled-cluster study of the ionic-neutral curve crossing LiF

“…The single reference based ansatz (SRMRCC) of Adamowicz et al [43] and Piecuch et al [44] uses the conventional single-reference coupled cluster wavefunction ansatz…”

“…In this paper we report results for the SRCC based state specific ansatz of Oliphant and Adamowicz (SRM-RCC) [43] and Piecuch et al [44] and the MRexpT ansatz of Hanrath [45,46] in application to lithium fluorine (LiF). The SRMRCC ansatz can be interpreted as a conventional SRCC method with additional (incomplete) higher excitations in the cluster operatorT and corresponding projections to span an MR space upon linearization of the exponential.…”

Dissociating N2: a multi-reference coupled cluster study on the potential energy surfaces of ground and excited states

“…Another category is not based on the SUMRCC or FSMRCC approaches but relies on extensions of the single-reference ansatz. Among these are the CC linear response approach [38,39], the singlereference formalism based ansatz (SRMRCC) [40,41], later variants [42][43][44][45] and related methods [46,47]. Additionally, there are renormalized [48] and method of moment [49,50] coupled cluster methods as well as dressed configuration interaction (CI) based ansätze [51][52][53], reduced MRCC [54,55] and partially linearized reduced MRCC approaches [56,57].…”

A simple approach to the state-specific MR-CC using the intermediate Hamiltonian formalism

“…a MRCEPA or MR(SC) 2 CI) formalism is not an easy task 29,30 . If one lets aside the MRCC methods that attribute a specific role to a single reference 31 , a few state-specific strictly multi-reference CC methods have been proposed, one by one of the authors and collaborators 1 , another one by Mukherjee and coworkers 32 , a third one in a Brillouin Wigner context 33,34 . We return here on the first proposal which had only be tested on a single problem.…”