Abstract
A method recently proposed by Brown and Truhlar to correct large-scale configuration interaction calculations for incomplete dynamic correlation of electrons is reformulated using the formalism of the double many-body expansion method. Rather than making the assumption that large-basis-set multireference configuration interaction calculations including all single and double excitations from a complete-active space recover an approximately constant fraction of the external correlation, it is suggested that this fraction is approximately constant for the various n-body terms which arise in a cluster expansion of the total molecular energy. A new variant of this method in which the assumption of geometry-independent fraction of the external correlation is relaxed for the diatomics is also examined. Results are reported for the ground electronic states of the H3 and HO2 systems.
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