Definition of eigenproblems suited to intermolecular perturbation theory

We compare the properties of those published intermolecular perturbation th eories whose behavior to infinite order is understood. From both fundamenta l and practical viewpoints we conclude that none is completely satisfactory . Since our current, detailed understanding of these theories is derived fr om the eigenproblems upon which they are based, we argue that it is critica l that any new perturbation theory be formulated as an eigenproblem. Since the eigenproblem upon which both the Amos-Musher (VI) and Polymeropoulos-Ad ams perturbation theories are based may be derived by the localized wave (L W) function method, and since both of these perturbation theories can give the ground-state energy exactly when carried to infinite order, we propose that the LW method is suitable for formulating eigenproblems upon which new perturbation theories may be based. We illustrate how the LW method may be used by deriving a new intermolecular perturbation theory, designed to be more accurate than the AM theory at large separations. Calculations on LiH show that this is the case, but that the theory becomes unsatisfactory at s mall separations. (C) 1999 John Wiley & Sons, Inc.