Comparative study of the nonlinear optical properties of extended π-electron structures obtained by ab initio and INDO methods: Hexapentaene, hexadiyne, and divinylacetylene

The theoretical investigation reported here has two objectives: (i) to compare the values of polarizability α and second hyperpolarizability γ obtained by the semiempirical INDO method with those obtained by the ab initio method, both utilizing the derivative approach, and (ii) to study the dependence of the α and γ on the nature of the extended π-electron structure in a group of compounds such as the six carbon atom molecules hexapentaene, hexadiyne, and divinylacetylene. First, to examine the dispersion and basis set effects, frequency-dependent α and different γ terms, responsible for various third-order nonlinear optical effects, are calculated for hexadiyne by the ab initio time-dependent Hartree-Fock method employing both STO-3G and 4-31G basis sets, the latter augmented with p and d semidiffuse functions on carbon. The coupled perturbed Hartree-Fock analytical method is used to compute the various derivatives. Compared to the extended basis results the minimal basis set, STO-3G, is found to heavily underestimate the values of α and γ. The zero-frequency limit static α and γ values for hexapentaene, hexadiyne, and divinylacetylene are calculated by the ab initio method using STO-3G and an augmented 6-31G basis set. The INDO method is used with the traditionally used minimal basis set involving a single ζ-function to calculate the α and γ values. The required derivatives for this calculation are obtained by the numerical finite field method. For each molecule, the INDO method gives the α and γ values which are significantly lower than those obtained with the extended basis set, augmented 6-31G, but comparable to the results obtained with the minimal basis set, STO-3G, in the ab initio calculation. In the group of six-carbon straight-chain molecules, α shows an increasing trend with the number of π-electrons but no definite trend is observed for the γ values.