Self-consistent field low rank perturbation method

The LRP-method is applied to the generalized perturbed eigenvalue equation, where the solution of the reference unperturbed equation is known. This me thod is generalized to the SCF approach. A simple case of rank one perturba tion is considered. It is shown that the operation count required to perfor m a single SCF iteration is of the order O(n(2)), where n is the order of t he matrices considered. This operation count is essentially independent of the magnitude of the perturbation. In addition, the number of SCF iteration s increases very slowly with the magnitude of the perturbation. The SCF LRP method can be applied to those problems where the rank of the perturbation is relatively small. In particular, it can be applied to localized perturb ations, where only a few perturbation matrix elements are nonzero. Such are , for example, substitution of an atom in a molecule by a heteroatom, forma tion or breaking of a chemical bond, etc.