Effect of solid-state interactions on the vibrational hyperpolarizability of all-trans polyacetylene chains from ab initio Hartree-Fock calculations

Coupled perturbed Hartree-Fock/6-31G finite oligomer cluster calculations are carried out, which show that solid-state interactions lead to a large decrease in the vibrational hyperpolarizability of all-trans polyacetylene. The vibrational property behaves similarly to the corresponding electronic property with increasing cluster size. Low-frequency collective and interchain modes are very significant for small oligomers, but the high-frequency vibrations that are most important for a single chain become dominant as the chains are lengthened. Since the solid-state interactions do not modify the geometry of the individual chains the decrease of the vibrational and electronic hyperpolarizabilities in polyacetylene is due to direct electronic effects.