“…In the other approach the optical susceptibility is regarded as arising from bond polarizabilities. The bond polarizability approach has been widely used to study the optical properties of crystals and has been applied, for example, to the linear susceptibility [2 to 81, the elasto-optic effect [2,9 to 111, the second-and third-order nonlinear optical susceptibilities [3,12 to 181, the linear and quadratic electro-optic effects [19,201, the first-and second-order Raman effects [36], the detection of very small atomic displacements by polarization methods [21], and the first-and second-order strain derivatives of electronic dielectric constants [22]. Many of the theories were applied to alkali halides.…”