Anharmonicity contributions to the vibrational first and second hyperpolarizability of para-disubstituted benzenes

In this Letter, we report on the anharmonicity contributions to the static vibrational first and second hyperpolarizability of para-disubstituted benzenes. MP2 and CCSD methods are used to determine the properties in question and, for 4-aminobenzaldehyde, we estimate the nuclear relaxation and electronic contribution to the longitudinal first hyperpolarizability at the CCSD(T)/aug-cc-pVTZ level of theory. This Letter demonstrates the importance of anharmonic contributions encompassed in the nuclear relaxation first and second hyperpolarizability. Moreover, it also points up the weakness of the double harmonic model which, in some instances, yields the incorrect sign of the vibrational contribution.