ORTHOGONALLY SPIN-ADAPTED STATE-UNIVERSAL COUPLED-CLUSTER FORMALISM -IMPLEMENTATION OF THE COMPLETE 2-REFERENCE THEORY INCLUDING CUBIC ANDQUARTIC COUPLING TERMS

The complete orthogonally spin-adapted Hilbert-space (or state-univers al) coupled-cluster (CC) theory involving singly and doubly excited cl usters for a model space spanned by two closed-shell configurations is considered. Explicit expressions for the previously neglected cubic a nd quartic coupling terms are derived using diagrammatic methods of ma ny-body perturbation theory and graphical methods of spin algebras. Th e resulting formalism has been implemented and the general purpose ab initio program has been written using newly developed procedure for im proving the convergence of the reduced linear equation method. Results of the pilot calculations for the two lowest singlet states of the mi nimum basis set and double zeta plus polarization basis H4 models as w ell as for the CH2 molecule at equilibrium and displaced geometries ar e presented and compared with the available single-reference CC and co nfiguration interaction data. They indicate negligible role of the cub ic and quartic coupling terms, which justifies the validity of approxi mations considered so far.