Static first hyperpolarizability of small all-trans polymethincimine oligomers. Basis set and electron correlation effects

Ab initio calculations, which include the electronic correlation effects, of the static longitudinal first hyperpolarizability, β_L, of small all-trans polymethineimine (PMI) chains (from monomer to trimer) have been performed with a large panel of basis sets. The electron correlation corrections were included via the Møller-Plesset partitioning and the Coupled-Cluster method. We show that the basis set effects are smaller when using correlated techniques than at the Hartree-Fock level. Moreover, the Hartree-Fock approximation does not reproduce the correct sign of β_L even for the dimer and the trimer. Addition of d functions on carbon and nitrogen atoms is important in order to obtain a quantitative evaluation of β_L, while the use of supplementary p functions on hydrogen atoms does not lead to significant variations. When polarization functions are added, the amplitude of β_L decreases by going from a double-ζ to a triple-ζ basis set. Within the Møller-Plesset scheme, the second-order correction is by far the largest and is responsible for the sign change. However, contrary to the third-order correction, the fourth-order correction considerably changes the β_L amplitude due to important single and triple contributions. In the Coupled-Cluster approaches, the inclusion of the singles strongly damps the correction due to the doubles, the inclusion of the triples within the CCSD(T) approach modulating it. Comparisons with other works demonstrate that the correlation effects are large in PMI oligomers. We conclude that the 6-31G basis set with d polarization functions on the carbon and nitrogen atoms, used in the Møller-Plesset partitioning consistent with electron - electron interactions through the fourth order, is suitable for the investigation of small PMI chains and would deserve to be considered in the study of longer chains.