Calculation of macroscopic first-, second-, and third-order optical susceptibilities for the urea crystal

Macroscopic first-, second-, and third-order susceptibilities of the urea crystal are calculated using static and frequency-dependent ab initio molecular (hyper)polarizabilities at the self-consistent field (SCF) and second-order-Møller-Plesset (MP2) levels. Environmental effects are taken into account using a rigorous local-field theory and are compared with the anisotropic Lorentz field factor approximation. The electric field arising from the permanent dipoles of the surrounding molecules is explicitly taken into account using a self-consistent approach. The dipole moment and the first hyperpolarizability are particularly strongly dependent on this field, but the crystal structure leads to a fortuitous cancellation of the field effect on the second-order susceptibility. The experimental linear susceptibility is accurately reproduced, while differences remain for the quadratic susceptibility. Dispersion curves for the first-order susceptibility, and results for quadratic electrooptic effect (QEO), electric-field-induced second-harmonic generation (EFISH), and third-harmonic generation (THG) experiments are predicted. The (hyper)polarizabilities of a linear dimer of urea molecules are calculated and used to estimate the effect of hydrogen bonding on the susceptibilities, which proves to be small. Semiempirically calculated (hyper)polarizabilities methods yield unreliable results for the susceptibilities compared with those from the ab initio method. This deficiency can be overcome by recourse to additional experimental data.