Is GVB-CI superior to CASSCF?

Abstract

Presently the most reliable approach for the study of reaction pathways where chemical bonds are broken and formed is to carry out CASSCF calculations followed by corresponding multireference perturbation or CI treatments. The latter step generally relaxes the “antibonding character” of the CASSCF results. In this study we demonstrate that similar results can be well approximated by using a less optimized MCSCF method and not performing the multireference perturbation or CI step at all. This is accomplished by performing a complete CI calculation within the active orbital space of the generalized valence bond perfect pairing (GVB-PP) model. The local bond/antibond character of the orbital space of the GVB-PP method also allows development of 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 bond breaking cases found in chemical reactions. This algorithm executes at a speed proportional to N where Np is the number of localized electron pairs in the active space. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 999–1008, 1999