The linear and nonlinear optical response properties of crystalline DEANST, N,N-diethyl-4-(2-nitroethenyl) phenylamine, have been computed rigorously, using a large set of input data including modified neglect of differential overlap (MNDO) and nb initio theoretical estimates of molecular polarizability alpha and hyperpolarizabilities beta and gamma, and experimentally measured refractive indices. The currently available microscopic expression for the third-order susceptibility tensor chi((3)) of a molecular crystal has been corrected to include all possible frequency combinations in the cascading term. Calculations of linear response based on theoretical polarizabilities alpha fail to reproduce the experimental refractive indices of the crystal, and hence an effective polarizability tensor has been derived from the measured refractive indices and used in the computation of the local electric field. The calculation of chi((3)) based on the MNDO hyperpolarizabilities is in tolerable agreement with the experimental measurements of third-harmonic generation. Environmental effects on the molecular response play a key role in the accuracy of the calculation. (C) 1997 American Institute of Physics.