Theoretical investigation of the second harmonic generation χ(2) tensor in crystals

François Mairesse and Benoît Champagne

University of Namur, Theoretical Chemistry Lab, Unit of Theoretical and Structural Physical Chemistry, Namur Institute of Structured Matter, rue de Bruxelles, 61, B-5000 Namur


The research presented in this poster deals with the evaluation, by the methods of solid-state quantum mechanics, of the second harmonic generation (SHG) second-order nonlinear optical (NLO) susceptibility, χ(2), of crystals. It first concentrates on reference NLO crystals, like urea and potassium dihydrogenophosphate, assessing the impact of several parameters to obtain χ(2) components close to the experimental ones. The second aspect consists of an investigation of the structure-χ(2) relationships in organic, inorganic and organic salt crystals, as well as in metal-organic frameworks (MOFs). The calculations are performed at the density functional theory (DFT) level of approximation and computational parameters include: i) the localized gaussian basis set, ii) the DFT exchange-correlation functional (XCF), and iii) the SCF parameters of a CRYSTAL17 calculation (associated with the truncation of lattice summations, with the number of points for the integration in the first Brillouin zone, and the convergence thresholds).  On the XCF part, the Hartree-Fock exchange effects are studied more thoroughly, highlighting that a tuned range-separated hybrid XCF is mandatory to describe adequately the band gap on which the optical properties of a crystal depend directly. Geometrical structure considerations are also analyzed as χ(2) is highly sensitive on the nuclei organization.