Microscopic calculation of the third-order nonlinear optical susceptibility of DEANST crystal

The linear and nonlinear optical response properties of crystalline DEANST, $\mathrm{N,N}$ -diethyl-4-(2-nitroethenyl) phenylamine, have been computed rigorously, using a large set of input data including modified neglect of differential overlap (MNDO) and ab initio theoretical estimates of molecular polarizability α and hyperpolarizabilities β and γ, and experimentally measured refractive indices. The currently available microscopic expression for the third-order susceptibility tensor ${{\chi }}^{\text{(3)}}$ 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 α 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 ${{\chi }}^{\text{(3)}}$ 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.