Single-root multireference Brillouin-Wigner coupled-cluster theory: Applicability to the F2 molecule

Recently developed single-root multireference Brillouin-Wigner coupled-cluster (MR BWCC) theory, which deals with one state at a time while employing a multiconfigurational reference wave function, is applied to the ground state of the F2 molecule using a two-determinant reference space at the level of the CCSD approximation. The method represents a brand-new coupled-cluster (CC) 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,1d] 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.