Electron correlation effects upon the static (hyper)polarizabilities of push-pull conjugated polyenes and polyynes

Ab initio calculations, including the electron correlation effects via the Moller-Plesset partitioning, of the static longitudinal polarizability (alpha(L)), first and second hyperpolarizabilities (beta(L) and gamma(L)) of small push-pull molecules have been carried out with the 6-31G and 6-31G* basis sets. Two types of conjugated segment (polyacetylene and polyyne) were used as linker between the nitro and amino fragments, chosen as acceptor and donor groups. For small chains, the triply bonded structures lead to larger gamma(L) than the doubly bonded structures, although the alpha(L) and beta(L) values order in the opposite direction. The second-order Moller-Plesset (MP2) corrections are essential to obtain correct estimates of the hyperpolarizabilities and are generally much larger than the MP3 and MP4 contributions. Geometry optimizations were carried out with both basis sets within the Hartree-Fock and MP2 schemes; the MP2 geometries lead to smaller bond length alternations and thus larger (hyper)polarizabilities. For alpha(L) a careful choice of the geometry is more crucial than the use of polarization functions during the electronic state calculations; however, for the hyperpolarizabilities, both the geometry and the basis set are of importance. (C) 1997 John Wiley & Sons, Inc.