Towards the multireference Brillouin–Wigner coupled-clusters method with iterative connected triples: MR BWCCSDT-α approximation

Abstract

We developed and implemented an approximation of the state-specific Brillouin-Wigner coupled-cluster method with singles, doubles, and triples, called MRBWCCSDT-alpha, for a general number of closed- and open-shell reference configurations. The accuracy of the method is assessed on the calculation of the oxygen molecule in the X3sigma(g-), a1delta(g), and b1sigma(g+) states and the results of this multireference treatment are compared with previous MRBWCCSD results and with those obtained by the doubly ionized similarity transformed equation-of-motion CCSD and multireference configuration interaction methods and with experimental spectroscopic data. Explicit tests of the size-extensivity of the MRBWCCSDT-alpha method with iterative size-extensivity correction are also performed.

Singlet-triplet gaps in substituted carbenes predicted from block-correlated coupled cluster method

“…However, to achieve quantitative accuracy, the CASSCF wave function must be augmented by dynamic correlation. A number of approaches that can incorporate dynamic correlation have been established, including (1) multireference CI with single and double excitations (MR-CISD) [3] ; (2) multireference perturbation theory (MRPT), such as CASSCF-based second-order perturbation corrections (CASPT2) [4,5] or multiconfigurational quasidegenerate perturbation theory [6] ; (3) multireference coupled cluster (MRCC) methods [7][8][9][10][11][12][13][14][15][16] . Among these approaches, the first two are the most commonly used theoretical methods, while MRCC methods have not been established as practical tools for routine uses, although they are expected to provide more accurate results than the MRCI or MRPT method with similar computational costs.…”

Multireference Coupled-Cluster Methods: Recent Developments

“…Recently, the connected triple excitations have been included in the MR BWCC theory [157][158][159], resulting in a considerable increase in the accuracy of the method thanks to a more precise description of dynamical correlation effects, similarly as in the SR case. As already mentioned, the main shortcoming of BWCC is the lack of the exact size-extensivity, only partly remedied by the non-iterative correction [142].…”

“…Recently, the connected triple excitations have been included in the MR BWCC method [157][158][159], considerably increasing its accuracy thanks to a more precise description of dynamic correlation effects, similarly as in the SR case, where one also has to account -at least approximately -for connected triply-excited clusters in order to achieve the chemical accuracy. For a recent detailed account of BW MBPTbased approaches we refer the reader to a forthcoming monograph by Hubač and Wilson [141].…”

Multireference coupled-cluster Ansatz

“…The MRexpT method of Hanrath [112,[139][140][141][142] and the C-conditions method of Li and Paldus [92][93][94][95][96][97] seem to offer a promising solution of the first problem. On should also point out that the incomplete model spaces and the C-conditions can also be used in the state-selective framework -in the BW-CC [119] and [60,151], who approximated the core-virtual bi-excited amplitudes by perturbation theory and obtained very good results with a greatly reduced number of variables in the nonlinear CC equations. In fact, this core-virtual bi-excited amplitudes are, through the first order of perturbation theory, independent of µ and one may contemplate to determine them nonperturbatively from MRCC equation assuming that they are the same for all µ's through all orders [152].…”

“…Some of these corrections [105] have turned out to give good results in practical applications [114][115][116][117]. The good accuracy achieved in recent applications of the BW-CC method was possible also due to the development of codes accounting for triple excitations [119][120][121]. Analytic gradients for the BW-CC method have also been developed [122].…”