SINGLE-ROOT MULTIREFERENCE BRILLOUIN-WIGNER COUPLED-CLUSTER THEORY - ROTATIONAL BARRIER OF THE ETHYLENE MOLECULE

Recently developed single-root multireference Brillouin-Wigner coupled cluster (MR BWCC) theory is applied to study the rotational barrier o f the ethylene molecule. The method belongs to a broad family of state -selective coupled-cluster (CC) methods and may be considered as a bri dge connecting the single-reference and multireference CC theories. In a highly nondegenerate case, it becomes identical with the standard s ingle-reference CC method while in a quasi-degenerate case gives resul ts very close to the Hilbert space MR CC method. The method switches b etween the two cases in a continuous way, providing thus smooth potent ial energy surfaces, not plagued by intruder states. The rotation abou t the double bond in ethylene represents a typical two-state problem. To demonstrate abilities of our approach, we study the rotation barrie r using the single-root MR BWCC theory at the CCSD level of approximat ion and the results are compared with other CC methods.