PROPERTY EVALUATION USING THE 2-REFERENCE STATE-UNIVERSAL COUPLED-CLUSTER METHOD

The recently developed and implemented orthogonally spin-adapted state -universal (SU) coupled-cluster (CC) theory using a model space spanne d by two closed-shell configurations and involving singly and doubly e xcited clusters (SU CCSD) is applied to calculate static properties of a few typical quasidegenerate systems, for which the range of quaside generacy can be continuously varied by changing their geometries. Elec trostatic multipole moments and polarizabilities are calculated for th e two lowest totally symmetric singlet states of the so-called H4 mode l consisting of two interacting hydrogen molecules in various geometri cal arrangements and for methylene at equilibrium geometry. In both ca ses, the double-zeta plus polarization basis set is employed and the p roperties are evaluated by the finite-field method. We discuss the rol e of orbital relaxation in the SU CC property calculations and compare our results with available single-reference (SR) CC, many-body pertur bation theory, and configuration interaction (CI) data, including the full CI results providing the exact solution for the given models. The studied systems enable us to examine several important aspects that a re encountered in property calculations when using the SU CC approach. In particular, the strongly degenerate region of the H4 model provide s us with several physically interesting situations, involving broken- symmetry solutions and a wrong sign or a wrong order of magnitude of t he multipole moments at the Hartree-Fock (HF) or even SR CC level of a pproximation. Our results indicate that SU CCSD provides accurate valu es for various electrostatic properties in both degenerate and nondege nerate regimes, regardless of whether the relaxed or nonrelaxed orbita ls are employed. At the same time, it gives very good property values for excited states. Finally, even when HF or SR CCSD results are quali tatively wrong due to the symmetry breaking, SU CCSD is capable of cor recting this behavior.