Multi-reference-state perturbation theory for computation of potential-energy surfaces

The implementation of multi-reference-state Rayleigh-Schrödinger perturbation theory for the evaluation of potential-energy surfaces is reviewed. The organization of the computation and the basic algorithms are outlined. A major difficulty is the quadratic increase in computing sources as the reference space is expanded. Truncating the space of nonreference determinants is suggested as a solution for the problem. The method applies a numerical cutoff criterion for the weights of the first-order wave function. The truncation reduces the computing time significantly with negligible sacrifice in the quality of the results.This paper was presented at the international conference on The Impact of Supercomputers on Chemistry, held at the University of London, London, UK, 13–16 April 1987