The high conductivity, fast nonlinear response, and charge storage in conjugated polymers reflects their Pi-electrons, and especially Pi-electron excitations. The frontier orbitals in sigma-conjugated polysilane polymers lead to similar models. We propose extension of Pariser-Parr-Pople theory via diagrammatic valence bond methods to obtain exact dynamic nonlinear coefficients in conjugated molecules or in polymeric fragments. This novel approach takes advantage of the large but finite VB basis for fourteen or less Pi-electrons and of sparse-matrix methods. We obtain, for the first time, exact NLO coefficients for interacting electrons and demonstrate major differences with current approximations. Extrapolations to polymers are proposed, with special emphasis on the frequency dependence of the responses. This grant is jointly supported by the Division of Materials Research and Chemistry.
