Is GVB-CI superior to CASSCF?

Presently the most reliable approach for the study of reaction pathways whe re chemical bonds are broken and formed is to carry out CASSCF calculations followed by corresponding multireference perturbation or CI treatments. Th e latter step generally relaxes the "antibonding character" of the CASSCF r esults. In this study we demonstrate that similar results can be well appro ximated by using a less optimized MCSCF method and not performing the multi reference perturbation or CI step at all. This is accomplished by performin g a complete CI calculation within the active orbital space of the generali zed valence bond perfect pairing (GVB-PP) model. The local bond/antibond ch aracter of the orbital space of the GVB-PP method also allows development o f a fast, but robust, Bethe-Goldstone algorithm, which reconstructs the CI energy to an accuracy of a few tenths of a millihartree for most types of b ond breaking cases found in chemical reactions. This algorithm executes at a speed proportional to N-p(4), where Np is the number of localized electro n pairs in the active space. (C) 1999 John Wiley & Sons, Inc.