Assessment of the single-root multireference Brillouin–Wigner coupled- cluster method: Test calculations on CH2, SiH2, and twisted ethylene

Abstract

Recently developed single-root multireference Brillouin–Wigner coupled-cluster (MR BWCC) theory, which belongs to a broad family of state-selective multireference coupled-cluster methods, has been implemented in the ACES II program package at the CCSD level of approximation. The method represents a new approach to quasidegenerate problems, which is able to continuously switch between the single-reference CC in a nondegenerate situation and the Hilbert-space MRCC in a degenerate case. An assessment of the method has been carried out by means of a comparison with the full configuration interaction (CI) treatments of CH2, SiH2, and twisted ethylene diradicals. The problem of size-extensivity is discussed.

Communication: Application of state-specific multireference coupled cluster methods to core-level excitations

“…31 In all calculations BW-MRCCSD energies were calculated without a posteriori correction. 25 Our EOMCC and MRCC results are presented in Table I. For H 2 O, we were not able to converge the EOM-CCSDT (EOMCC with singles, doubles, and triples) calculations for all basis sets employed.…”

“…In the present work, we apply state-specific multireference CC (SS-MRCC) methods 25,26 to the CL states. The main idea of the proposed formalism is to employ a multidimensional model space, which includes basic non-dynamic correlation effects relevant to a CL state of interest.…”

“…For specifying CMS we use the M 0 (n el , n orb ) convention, where n el and n orb refer to the number of active electrons and active orbitals, respectively. In our calculations, we used state-specific Brillouin-Wigner and Mukherjee MRCC approaches 25,26 (BW-MRCC and Mk-MRCC for short). Although Hilbert-space MRCC methods have been mostly applied to quasidegenerate ground states and lowlying excited states, the model space concept is flexible enough to embrace the class of states dominated by corevalence excitations.…”

Adiabatic state preparation study of methylene

“…[59][60][61][62]). We are in fact following our previous paper [34], where we used CH 2 as a benchmark for simulations of computations with the quantum FCI method based on the iterative phase estimation algorithm (IPEA).…”

The State-Universal Multi-Reference Coupled-Cluster Theory: An Overview of Some Recent Advances

“…In this paper, we focus on the genuine multi-reference CC (MRCC) theories, which are based on the concept of effective Hamiltonian acting in a multi-dimensional reference or model space [4,6,[8][9][10][11][12] and which use the exponential CC ansatz to parametrize the solutions of the generalized Bloch equation [8]. The state-specific (SS) MRCC methods [4,6], including the active-space CC approaches of Adamowicz, Piecuch, Bartlett, and their collaborators [13][14][15][16][17][18][19][20][21][22][23][24][25][26] and their more recent excited-state extensions [27][28][29], the SSMRCC approach of Mahapatra et al [30,31], and the highly promising Brillouin-Wigner MRCC approach [32][33][34][35][36][37], will not be discussed in this work, although the similarity of the wave function ansatz used in the genuine state-universal MRCC theory [4,6,, considered in this paper, and the form of the wave function exploited by the SSMRCC and Brillouin-Wigner MRCC approaches of Refs. 30, 31 and 32-37, respectively, should be noted.…”