P. Piecuch et J. Paldus, PROPERTY EVALUATION USING THE 2-REFERENCE STATE-UNIVERSAL COUPLED-CLUSTER METHOD, Journal of physical chemistry, 99(42), 1995, pp. 15354-15368
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.