Linear and nonlinear second order susceptibilities of molecular crystals

First and second order susceptibilities have been investigated in previous works [1,2,3] for molecular crystals presenting very interesting features for nonlinear optics. Among them, 3-methyl-4-nitroaniline (MNA), 3-methyl-4- nitroaniline-1-oxyde (POM) and urea. In the first one, stacking unit cells along the a-axis (parallel to the dipole moment of the constituting molecular pairs) yield a strong enhancement of the effective first hyperpolarizability of the cluster, while stacking along the other directions leads to the opposite behavior. In the case of POM, the effective β value is close to the molecular response of an isolated molecule whereas all other tensor components vanish by symmetry upon packing. The urea crystal presents a supplementary feature related to intermolecular interactions through hydrogen bonding. Its microscopic properties were evaluated employing the supermolecule approach by means of the TDHF/AM1 procedure and the macroscopic quantities were obtained by adopting the multiplicative scheme corrected for missing correlation effects by DFT and coupled cluster calculations. Such combination appears to be an efficient tool to investigate, at least at a semiquantitative level, the linear and NLO properties of molecular crystals.