Frequency-dependent polarizabilities and second hyperpolarizabilities of N₂

The static- and frequency-dependent dipole polarizabilities α and second hyperpolarizabilities γ of N₂ are calculated at different wavelengths using self-consistent field and multiconfigurational self-consistent field linear and quadratic response theory. Electron correlation is found to be important. With an active orbital space consisting of one correlating orbital for each occupied valence orbital excellent agreement is obtained with the experimental polarizabilities and hyperpolarizabilities. The dispersion of the hyperpolarizability which previously has been determined from electric-field-induced second harmonic generation experimental data is shown to give an inaccurate description in the low frequency region due to lack of flexibility in the fitting function. Our results stress that it is important to carry out calculations at frequencies where experiments are carried out to avoid extrapolation to the static limit. Basis set effects have been investigated. f functions are important whereas g functions have little effect. The vibrational averaging has been found to be of little importance.