GVB-RP: A reliable MCSCF wave function for large systems

We have developed a version of generalized valence bond (GVB) that overcomes the major weakness of the perfect pairing approximation without requiring a full transformation of the integrals at each step of the self-consistent orbital optimization. The method, called generalized valence bond-restricted pairing (GVB-RP), describes properly the dissociation of up to triple bonds and provides smooth potential energy surfaces for most chemical reactions. The wave functions obtained are a good starting point for more sophisticated computational techniques. The applicability of the method is illustrated with a few simple examples including multiple-bond dissociations, transition states for symmetry allowed, symmetry forbidden, and radical reactions, as well as reactions at a transition-metal center. The cost of the method compares well with other self-consistent correlated techniques.