Analysis of the vibrational static and dynamic second hyperpolarizabilities of polyacetylene chains

The RHF/6-31G longitudinal vibrational second hyperpolarizabilities of increasingly longer polyacetylene chains have been computed within the double harmonic approximation. The [alpha(2)](0,0) contribution converges rapidly with chain length to an asymptotic limit 30% larger than its electronic counterpart. The [mu beta](0,0) term converges much more slowly. It is only 4% of the electronic term for C16H18, the longest oligomer considered here, but the ratio is growing rapidly with chain length.