APPROXIMATELY EXTENSIVE MODIFICATIONS OF THE MULTIREFERENCE CONFIGURATION-INTERACTION METHOD - A THEORETICAL AND PRACTICAL ANALYSIS

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.