DOUBLE HARMONIC VIBRATIONAL 2ND HYPERPOLARIZABILITIES OF POLYYNE CHAINS

The RHF/6-31G longitudinal vibrational second hyperpolarizabilities of increasingly longer polyyne chains have been computed within the doub le harmonic oscillator approximation. Similarly to polyacetylene, the largest contribution comes from the [alpha(2)](0,0) term. About 95% of the latter arises from what we call the quasi-ECC mode. It converges rapidly with chain length to an asymptotic limit slightly (7%) smaller than its static electronic counterpart. The [mu beta](0,0) term is ne gligible. The inclusion of electron correlation within the MP2/6-31G s cheme leads to a decrease of the gamma(v)/gamma(e) ratio by a factor g oing from roughly 3 for the dimer to 2 for the tetramer. (C) 1998 Else vier Science B.V.