Ab initio studies of the dipole polarizabilities of conjugated molecules Part 1. Ethene and benzene

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We report an accurate and systematic ab initio SCF MO study of the molecular geometry and the static dipole polarizability of ethene and benzene. The sensitivity of polarizability calculation to the effects of the geometry optimizations, the choice of basis set and to the inclusion of electron correlation at the MP2 level was studied. The basis sets were constructed by extension of the STO/6-31G of STO/6-311G with diffuses and p functions and multiple d and p polarization functions, giving the STO/6-31 + (nd,mp) and STO/6-311 + G(nd,mp) scheme. We show that these basis sets, with n = m = 3, lead to SCF and MP2 polarizability values for ethene and SCF polarizability for benzene that are in excellent agreement with the available theoretical and experimental data. We also found that the effect of MP2 correlation on the polarizability calculations, which are greater in ethene than benzene, decrease when the number of polarization functions is increased in the diffuse basis set.