Nonlinear susceptibilities of organic crystals are usually calculated from the molecular nonlinearities(hyperpolarizabilities) and the crystal structure data within the oriented gas model approximation. The effect of intermolecular interactions on hyperpolarizabilities, which is neglected in the approximation, is estimated quantitatively by using an ab initio molecular orbital(MO) method. Energy minimum crystal structures of typical organic nonlinear optical materials are calculated by an empirical atom-atom pairwise potential method.