The CNDO/VS sum-over-states (SOS) method has been used to calculate the scalar components of the second-order hyperpolarizability tensors appropriate to d.c.-field-induced second-harmonic generation (DC-SHG) at 1.064 nm and third-harmonic generation (THG) at 1.064 and 1.9 nm. The molecules considered are benzene, aniline, nitrobenzene and p-nitroaniline (PNA).

The above calculations have been carried out using both the singly excited configuration interaction (SCI), and the singly and doubly excited configuration interaction (SDCI) models. Their suitability for evaluating third-order effects has been assessed, by extensively testing the convergence of the perturbation sum for the above molecules, although it must be noted that the results obtained are only semiquantitative. Furthermore, the importance of the charge-transfer state of PNA has also been assessed and the frequency dependence of its contribution investigated.