P. Piecuch et J. Paldus, APPLICATION OF HILBERT-SPACE COUPLED-CLUSTER THEORY TO SIMPLE (H-2)(2) MODEL SYSTEMS .2. NONPLANAR MODELS, Physical review. A, 49(5), 1994, pp. 3479-3514
In this series, the recently developed explicit formalism of orthogona
lly spin-adapted Hilbert space (or state universal), multireference (M
R) coupled-cluster (CC) theory, exploiting the model space spanned by
two closed-shell-type reference configurations, is applied to a simple
minimum-basis-set four-electron model system consisting of two intera
cting hydrogen molecules in various geometrical arrangements. In this
paper, we examine the nonplanar geometries of this system, generally r
eferred to as the T4 models, and their special cases designated as P4
and V4 models. They correspond to different cross sections of the H-4
potential-energy hypersurface, involving the dissociation or simultane
ous stretching of two H-H bonds. They involve various quasidegeneracy
types, including the orbital and configurational degeneracies, the two
fold degeneracy of the ground electronic state and interesting cases o
f broken-symmetry solutions. We employ the CC with singles and doubles
(SD) approximation, so that the cluster operators are approximated by
their one- and two-body components. Comparing the resulting CC energi
es with exact values, which are easily obtained for these models by us
ing the full configuration-interaction method, and performing a cluste
r analysis of the exact solutions, we assess the performance of variou
s MRCC Hilbert-space approaches at both linear and nonlinear levels of
approximation, while a continuous transition is being made between th
e degenerate and nondegenerate or strongly correlated regimes. We eluc
idate the sources and the type of singular behavior in both linear and
nonlinear versions of MRCC theory, examine the role played by various
intruder states, and discuss the potential usefulness of broken-symme
try MRCCSD solutions.