A simple variant of perturbation theory is used to correct reference states of a general multiconfigurational character. The full solution of an active space is not required, and no iterative procedure is applied to construct the resolvent operator. The perturbed wave function is expanded in a complete set of determinants from which the reference function is projected out, and the overlap between projected determinants is handled by an explicit, analytic inversion of the overlap matrix. (C) 2003 American Institute of Physics.