ORTHOGONALLY SPIN-ADAPTED STATE-UNIVERSAL COUPLED-CLUSTER FORMALISM -IMPLEMENTATION OF THE COMPLETE 2-REFERENCE THEORY INCLUDING CUBIC ANDQUARTIC COUPLING TERMS
P. Piecuch et J. Paldus, ORTHOGONALLY SPIN-ADAPTED STATE-UNIVERSAL COUPLED-CLUSTER FORMALISM -IMPLEMENTATION OF THE COMPLETE 2-REFERENCE THEORY INCLUDING CUBIC ANDQUARTIC COUPLING TERMS, The Journal of chemical physics, 101(7), 1994, pp. 5875-5890
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.