Fully variational optimization of modern VB wave functions using the CASVB strategy

We outline the CASVB strategy, which may be used either to generate very compact modern valence bond representations of CASSCF wave functions or to optimize general types of modern VB wave function. A simple algorithm is presented for the elimination of redundant or constrained parameters from the appropriate second-order optimization problems. Selected results for three systems—benzene, the allyl radical, and LiH—are used to illustrate various ways in which CASVB procedures can be used to perform fully variational optimizations of nonorthogonal orbitals and structure coefficients in modern VB wave functions.