Enhanced second-order treatment of electron pair correlation

Ab initio calculations have been performed for F-2, HCCH, H2O, HF, (HF)(2), and (H2O)(2), comparing certain electron pair correlation methods, or methods for doubly substituted configurations. In these model systems, the reweighting of substituted configurations that occurs beyond a second-order perturbative treatment of electron correlation can be partly built into the second-order analysis in a computationally trivial step. Specific means for doing this are explored, and they offer improvement in certain cases or else very little change. A consistent improvement in the correlation energy when judged against treatment with double substitution coupled cluster theory for the test species is obtained through one of these schemes. (C) 2000 John Wiley & Sons, Inc.