Contribution of molecular displacements to linear and nonlinear electric susceptibilities of molecular crystals

Expressions are derived for additional contributions to the linear, quadratic, and cubic electric susceptibilities of molecular crystals that arise when molecules are displaced by the applied electric field. The contributions depend on quantities related to the infrared intensity of lattice vibrations, to the Raman intensity of lattice vibrations, and to the intensity of hyper-Rayleigh scattering. Some nonlinear contributions are zero except for response to a static electric field applied directly or produced by optical rectification. There are also contributions from averaging the susceptibilities in the equilibrium structure over the lattice modes.