Pg. Szalay et Rj. Bartlett, APPROXIMATELY EXTENSIVE MODIFICATIONS OF THE MULTIREFERENCE CONFIGURATION-INTERACTION METHOD - A THEORETICAL AND PRACTICAL ANALYSIS, The Journal of chemical physics, 103(9), 1995, pp. 3600-3612
The extensivity error of configuration interaction (CI) is well unders
tood and unlinked diagram corrections must be applied to get reliable
results. Besides the well known a postepiori Davidson-type corrections
, several methods attempt to modify the CI equations a priori to obtai
n nearly extensive results, while retaining the convenience of working
in a configuration space. Such unlinked diagram corrections are parti
cularly important for multireference cases for which coupled-cluster (
CC) calculations, which require a many-body, integral-based calculatio
n, are more difficult. Several such multireference methods have been p
resented recently, ranging from the multireference linearized coupled
cluster method (MR-LCCM), averaged coupled pair functional (MR-ACPF),
through various quasidegenerate variational perturbation theory (QD-VP
T), MR-coupled electron pair method (MR-CEPA) to size-consistent, self
-consistent, selected CI [(SC)(2)SCi]. We analyze all of these methods
theoretically and numerically, paying particular attention to the new
multireference averaged quadratic CC method (MR-AQCC), and demonstrat
e its comparative quality of performance even when using small referen
ces spaces. We consider several demanding molecular examples that bene
fit from a multireference description, Like bond stretching in H2O; N-
2 and C-2; the insertion of Be into H-2; and the singlet-triplet split
ting in CH2. We also investigate the extensivity error. (C) 1995 Ameri
can Institute of Physics.