A NEW MCSCF METHOD

Abstract

We have recently developed a multi-configuration self-consistent field (MCSCF) method which has distinct advantages over previously implemented approaches to the solution of MCSCF equations, especially with regard to convergence behavior. This work contains considerable formal development in order to achieve a clear and general formulation of the method. A key feature is the introduction of one-and two-electron shell replacement operations. The operator manipulations that emerge serve as the vehicle for a transparent and relatively simple development. In particular, considerations based on molecular symmetry are incorporated at the outset and retained throughout without significant additional effort. Such a systematic consideration of symmetry is in marked contrast to pervious formal discussions of MCSCF methodology which have totally ignored symmetry or appended it as an afterthought. In our method the self-consistent field condition is achieved as a result of successive unitary transformations on both the orbital expansion coefficients and the state expansion treatment are distinctive in that no eigenvalue or secular equations are involved. This feature enhances the efficiency with which our method can be implemented. In addition, our approach yields quadratic convergence of the MCSCF optimization procedure in the general case. Past methods could not be shown to be reliably convergent in every situation, let alone quadratically convergent. A program implementing our method is currently under development, and exploratory calculations demonstrating the practical behaviour of the procedure will be discussed.