SINGLE-ROOT MULTIREFERENCE BRILLOUIN-WIGNER COUPLED-CLUSTER THEORY - ROTATIONAL BARRIER OF THE N2H2 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 t o study the rotational barrier of the N2H2 molecule. The method repres ents a brand new coupled-cluster (CC) approach to quasi-degenerate pro blems which combines merits of two approaches: the single-reference CC method in a nondegenerate case and the Hilbert space MR CC method in quasi-degenerate case. The method is able to switch itself from a nond egenerate to a fully degenerate case in a continuous manner, thus prov iding smooth potential energy surfaces. Moreover, in contrast to the H ilbert space MR CC theory, it does not contain the so-called coupling terms and in a highly nondegenerate case it reduces to a standard sing le-reference CC method. In order to better judge the abilities of our new approach, we study the rotation barrier of the N2H2 molecule at th e CCSD level and the results are compared with the single-reference CC SD and Hilbert space MR CCSD methods. The rotation of the N2H2 molecul e from a trans-to cis-conformer represents a typical two-state problem in which the weights of reference configurations can change from 0 to I in a continuous manner and, in contrast to the H-4 models, it repre sents a real system.