MOLECULAR-ORBITAL EXPRESSIONS FOR APPROXIMATE UNCOUPLED HARTREE-FOCK 2ND HYPERPOLARIZABILITIES - A PARISER-PARR-POPLE ASSESSMENT FOR MODEL POLYACETYLENE CHAINS

Various partitioning schemes to the uncoupled Hartree-Fock second hype rpolarizability and three of its approximations have been derived. One -electron eigenstate formula have been determined. The validity of the se schemes for studying polymeric systems has been assessed by investi gating the evolution with chain length of the longitudinal second hype rpolarizability per unit cell of model polyacetylene chains. This stud y of the size-consistency of the various contributions has been carrie d out within the Pariser-Parr-Pople method. It is shown that all the a pproximations and partitioning schemes work well for the smallest chai ns. The less alternating the electron distribution along the polymeric backbone, the better the approximations and partitioning schemes.