The externally corrected coupled cluster approach with four- and five-body clusters from the CASSCF wave function

Abstract

An externally corrected CCSDt (coupled cluster with singles, doubles, and active triples) approach employing four- and five-body clusters from the complete active space self-consistent field (CASSCF) wave function (denoted as ecCCSDt-CASSCF) is presented. The quadruple and quintuple excitation amplitudes within the active space are extracted from the CASSCF wave function and then fed into the CCSDt-like equations, which can be solved in an iterative way as the standard CCSDt equations. With a size-extensive CASSCF reference function, the ecCCSDt-CASSCF method is size-extensive. When the CASSCF wave function is readily available, the computational cost of the ecCCSDt-CASSCF method scales as the popular CCSD method (if the number of active orbitals is small compared to the total number of orbitals). The ecCCSDt-CASSCF approach has been applied to investigate the potential energy surface for the simultaneous dissociation of two O-H bonds in H2O, the equilibrium distances and spectroscopic constants of 4 diatomic molecules (F2(+), O2(+), Be2, and NiC), and the reaction barriers for the automerization reaction of cyclobutadiene and the Cl + O3 → ClO + O2 reaction. In most cases, the ecCCSDt-CASSCF approach can provide better results than the CASPT2 (second order perturbation theory with a CASSCF reference function) and CCSDT methods.

Addressing Strong Correlation by Approximate Coupled-Pair Methods with Active-Space and Full Treatments of Three-Body Clusters

“…The major breakthrough in the understanding of the superb performance of the ACP-D45 approach in a strongly correlated regime came in 1984 [125], followed by two other key articles published in 1991 [64] and 1996 [126]. By performing cluster analysis [124] of the PUHF wave function within the orthogonally spin-adapted framework, in which the PUHF state was assumed to be exact, and using the philosophy of the externally corrected CC methods [125,126,161,[198][199][200][201][202][203][204][205][206][207][208][209], the authors of Refs. [125,126] demonstrated that the T 4 cluster component extracted from PUHF with the help of the Thouless theorem and read into the CCD system, Eq.…”

Externally Corrected CCSD with Renormalized Perturbative Triples (R-ecCCSD(T)) and the Density Matrix Renormalization Group and Selected Configuration Interaction External Sources

“…The ecCCSD method has a long history and external sources, ranging from unrestricted Hartree-Fock [86][87][88][89] to CASSCF and CASCI [90][91][92][93][94] and, most recently, full configuration interaction quantum Monte Carlo, 95 have been used. One of the more successful applications of ecCCSD is the reduced multireference (RMR) CCSD method, 96,97 which uses a MRCISD wave function as the external source.…”

Externally corrected CCSD with renormalized perturbative triples (R-ecCCSD(T)) and density matrix renormalization group and selected configuration interaction external sources

“…The ecCCSD method has a long history and external sources, ranging from unrestricted Hartree-Fock [86][87][88][89] to CASSCF and CASCI [90][91][92][93][94] and, most recently, full configuration interaction quantum Monte Carlo, 95 have been used. One of the more successful applications of ecCCSD is the reduced multireference (RMR) CCSD method, 96,97 which uses a MRCISD wave function as the external source.…”