Bond length alternation effects on the static electronic polarizability and second hyperpolarizability of polyacetylene chains

The static electronic longitudinal polarizability (α) and second hyperpolarizability (γ) of all-trans planar polyacetylene chains have been computed at different levels of approximation by varying the bond length alternation (BLA) along the conjugated backbone. Although the restricted Hartree-Fock (RHF) and second-order Møller-Plesset (MP2) approaches exhibit the expected behavior, i.e., the smaller the BLA, the larger the γ, the density functional schemes based on the SVWN and B3LYP exchange correlation functionals behave in a totally different way. when BLA increases, γ first increases, attains a maximum, and then decreases. Moreover, the RHF approach is found to overestimate by a factor of 2-3 the BLA dependence of α with respect to the MP2 procedure.