Effects of molecular charge density polarization on Χ(1) and Χ(2): A computational study of 2-methyl-4-nitroaniline (MNA) and 4-(N,N-dimethylamino) -3-acetamidonitrobenzene (DAN)

The linear and second-order nonlinear optical susceptibilities of the MNA and DAN organic molecular crystals have been determined using a computationally cheap method, the Rigorous Local Field Theory, which consists in estimating the bulk properties from 1°) the corresponding molecular responses, evaluated using quantum chemistry methods, as well as 2°) the local and dressing electric fields, evaluated using a classical electrostatic interaction scheme. This study has highlighted several conditions for obtaining accurate theoretical results. First, the input molecular geometry should match the main features of the crystal structure. Then, the molecular properties should be evaluated at a correlated level since electron correlation effects are huge for the polarizability (α) and even more for the first hyperpolarizability (β). The recommended approach is to use static MP2 molecular responses together with the multiplicative scheme to describe frequency dispersion. The third important aspect is the impact of the chromophore electron density polarization due to surrounding molecules in the crystal lattice. This effect accounts for an enhancement of ca. 100-200% of β and thus influences strongly the calculated bulk Χ(2) tensor components.