Collect. Czech. Chem. Commun. 1998, 63, 1213-1222

Single-Root Multireference Brillouin-Wigner Coupled-Cluster Theory. Rotational Barrier of the Ethylene Molecule

Jozef Mášika, Pavel Macha, Ján Urbana, Martin Polášekb, Peter Babineca and Ivan Hubača,b

a Department of Chemical Physics, Comenius University, 842 15 Bratislava, Slovak Republic
b Faculty of Science, Silesian University, 746 01 Opava, Czech Republic


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.

Keywords: State-selective; Multireference; Coupled cluster; Hilbert space; Brillouin-Wigner; Single-root; Ab initio calculations; Quantum chemistry.