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 to study the rotational barrier of the N2H2 molecule. The method represents a brand new coupled-cluster (CC) approach to quasi-degenerate problems 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 nondegenerate to a fully degenerate case in a continuous manner, thus providing smooth potential energy surfaces. Moreover, in contrast to the Hilbert space MR CC theory, it does not contain the so-called coupling terms and in a highly nondegenerate case it reduces to a standard single-reference CC method. In order to better judge the abilities of our new approach, we study the rotation barrier of the N2H2 molecule at the CCSD level and the results are compared with the single-reference CCSD and Hilbert space MR CCSD methods. The rotation of the N2H2 molecule 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 represents a real system.