SINGLE-ROOT MULTIREFERENCE BRILLOUIN-WIGNER COUPLED-CLUSTER THEORY - APPLICABILITY TO THE F-2 MOLECULE

Recently developed single-root multireference Brillouin-Wi,oner couple d-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 Fr! molecule using a two-determinant refere nce space at the level of the CCSD approximation. The method represent s a brand-new coupled-cluster (CC) approach to quasidegenerate problem s which combines merits of two theories: the single-reference CC metho d in a nondegenerate case and the Hilbert space MR CC method in quasid egenerate case. The method is able to switch itself from a nondegenera te to a fully degenerate case in a continuous manner, providing thus s mooth potential energy surfaces. Moreover, in contrast to the Hilbert space MR CC approaches, it does not contain the so-called coupling ter ms and completely reduces to the standard single-reference CC method i n a highly nondegenerate region. Using a [4s, 3p, 1d] and [4s, 3p, 2d, 1f] basis sets, the calculated potential energy curves are smooth, di ssociate correctly and the results are compared with other available m ultireference techniques as well as experiment. (C) 1998 American Inst itute of Physics.