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An anharmonic vibronic oscillator model is set up and used to derive general expressions for nonlinear susceptibilities. The system is handled both classically and quantum-mechanically, and the results are shown to agree. It is shown that, when one of the interacting fields has a frequency close to that of an optical phonon, the nonlinear susceptibility is dominated by the same term in the anharmonic potential that is dominant in Raman scattering. This result agrees with the experiments and analysis of Faust and Henry. These arguments are extended to cover electrooptic coefficients, for which the model is consistent with the work of Kaminow and Johnston. On the basis of the anharmonic vibronic oscillator model, estimates are made of the magnitude of infrared electroreflectance effects and their counterpart, optical rectification. The conversion gain of such a rectifier is calculated. The model is also used to determine Raman or parametric gain, depending on whether the idler frequency is or is not close to the optical phonon frequency. Finally the model is used to compute the pyroelectric coefficient, which, it is shown, ought to be proportional to the specific heat, in agreement with recent measurements by Heiland and Ibach on zinc oxide.