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.

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Articles you may be interested inBlock correlated coupled cluster theory with a complete active-space self-consistent-field reference function: The formulation and test applications for single bond breaking On the ground states of CaC and ZnC: A multireference Brillouin-Wigner coupled cluster study J. Chem. Phys. 117, 9733 (2002); 10.1063/1.1516809 Brillouin-Wigner coupled cluster theory. Fock-space approachRecently developed single-root multireference Brillouin-Wigner coupled-cluster ͑MR BWCC͒ theory, which deals with one state at a time while employing a multiconfigurational reference wave function, is applied to the ground state of the F 2 molecule using a two-determinant reference space at the level of the CCSD approximation. The method represents a brand-new coupled-cluster ͑CC͒ approach to quasidegenerate problems which combines merits of two theories: the single-reference CC method in a nondegenerate case and the Hilbert space MR CC method in quasidegenerate case. The method is able to switch itself from a nondegenerate to a fully degenerate case in a continuous manner, providing thus smooth potential energy surfaces. Moreover, in contrast to the Hilbert space MR CC approaches, it does not contain the so-called coupling terms and completely reduces to the standard single-reference CC method in a highly nondegenerate region. Using a ͓4s,3p,1d͔ and ͓4s,3p,2d,1f ͔ basis sets, the calculated potential energy curves are smooth, dissociate correctly and the results are compared with other available multireference techniques as well as experiment.

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