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