Single-reference CCSD approach employing three- and four-body CAS SCF corrections: A preliminary study of a simple model

Abstract

Ž .The idea of correcting the single-reference coupled cluster CC method truncated at the Ž . pair cluster level CCSD by means of three-and four-body corrections coming from w Ž . some external source J. Paldus, J. Cızek, and M. Takahashi, Phys. Rev. A 30, 2193 1984 ;ˇŽ .x J. Paldus and J. Planelles, Theor. Chim. Acta 89, 13 1994 is explored at the ab initio level using the CAS SCF wave functions as a source of the triply and quadruply excited Ž . cluster amplitudes. The method referred to as CCSD᎐CAS is applied to three simple systems based on minimum basis set and double-zeta models of the BH molecule, in Ž . Ž . which we continuously vary, respectively, i the two electron repulsion, ii the electronic Ž . charge, and iii the internuclear separation in order to explore the performance of the method in quasi-degenerate situations. Both the energies and the higher than pair cluster amplitudes are compared with the corresponding exact full configuration interaction Ž . FCI results. The relative importance of the three-and four-body cluster components is also examined. In all cases considered, the CCSD᎐CAS method provides the best result.

Electronic structure of organic diradicals: Evaluation of the performance of coupled-cluster methods

“…In contrast, the ecCCSD methods [35][36][37][38][39][40][41][42] employ some external ͑i.e., non-CC or MBPT͒ wave function as a source of higher-than-pair clusters, such as a variational MR CISD ͑configuration interaction with singles and doubles͒ wave function in the case of the RMR CCSD method. [43][44][45][46] The external source wave function must be capable to properly describe the desired dissociation channel or the considered radical species, being based on a size-consistent zero-order wave function.…”

Multireference coupled‐cluster methods for ground and low‐lying excited states. A benchmark illustration on CH + potentials

“…For these and other reasons much attention has been lately given to the so‐called state selective or state specific (SS) versions of the SU CC method focussing on one state at a time. (Note, however, that the term SS has often been used even for SR‐type approaches that account for MR effects in one way or another; see, e.g.,27–33, etc.). Here, we shall understand by the SS approaches those methods that use the SU CC Ansatz and formalism, but modify it in such a way that the SU CC system of equations is solved anew for each state involved.…”

“…Moreover, the GMS SU CCSD approach was further modified by exploiting the idea of external corrections (ec) as in SR ecCCSD29–33, which leads to the reduced MR (RMR) CCSD method (see, e.g.,9, 10, 60–66 and references therein) and in the case of GMS SU CCSD to the so‐called ( N , M )‐CCSD method67. Here the M ‐reference GMS SU CCSD is corrected for higher order clusters by relying on M pertinent wave functions produced by an N ‐reference MR CISD ( M ≤ N ).…”

Method of Moments of Coupled-Cluster Equations: Externally Corrected Approaches Employing Configuration Interaction Wave Functions

“…The representative examples include the externally corrected CC methods of Paldus and co-workers [6,10,[39][40][41][42][43][44][45][46][47][48][49] (see, also, Ref. 50), the activespace CC approaches of Adamowicz, Piecuch, Bartlett, and their collaborators [14,19,[51][52][53][54][55][56][57][58][59][60][61][62], the orbital-optimized CC methods of Head-Gordon et al [23,24], the perturbative CC approaches based on the partitioning of the similarity-transformed Hamiltonian of Gwaltney and HeadGordon [25][26][27], the state-selective MRCC approach of Mahapatra et al [63,64], the BrillouinWigner MRCC approach of Hubač and co-workers [65][66][67][68][69][70], and the method of moments of coupledcluster equations (MMCC) of Kowalski and Piecuch [7,[16][17][18]…”

“…In this paper, we consider a possibility of approximating the wave function |Ψ that enters the MMCC energy correction by wave functions obtained in inexpensive CI calculations. In analogy to the externally corrected CC methods [6,10,[39][40][41][42][43][44][45][46][47][48][49][50], in which non-CC wave functions are used to provide information about higher-than-doubly excited clusters, we call the resulting MMCC schemes the externally corrected MMCC methods. 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 H 2 O.…”