Møller–Plesset evaluation of the static first hyperpolarizability of polymethineimine

A finite oligomer approach is used to compute the longitudinal component of the static electronic first-hyperpolarizability per unit cell (ΔβL(N)) of polymethineimine. The longitudinal first hyperpolarizability (βL) is calculated accounting for electron correlation effects via the Møller–Plesset partioning. The influences of the basis set size, of the order in electron–electron interactions up to which the electron correlation effects are accounted for and of the modification of the equilibrium geometry due to the inclusion of electron correlation are considered. Contrary to the Hartree–Fock level for which ΔβL(N) vs. N firstly presents a minimum then increases towards its polymeric value, at the MP2 level, ΔβL(N) evolves monotonically with chain length and its asymptotic value is 7.7 times larger than the CPHF results reaching the large value of 4.2×10−30 esu g−1 mol.