Elongation method and supermolecule approach for the calculation of nonlinear susceptibilities. Application to the 3-Methyl-4-Nitropyridine 1-Oxide and 2-Methyl-4-Nitroaniline crystals.

The linear and nonlinear optical properties of the 3-methyl-4-nitropyridine-1-oxyde (POM) and 2-methyl-4-nitroaniline (MNA) crystals are evaluated using the elongation method and the supermolecule approach. The elongation calculations adopt the finite field Hartree-Fock numerical scheme to get the static polarizability, first and second hyperpolarizabilities of large one-dimensional clusters, allowing a very accurate extrapolation of the crystal packing ratios to the infinite one-dimensional arrays. The supermolecule approach employs the analytical time-dependent Hartree-Fock scheme for evaluating the static and dynamic polarizability and first hyperpolarizabilities of smaller clusters. Most of the calculations are performed using the semiempirical AM1 Hamiltonian. Preliminary results are also carried out at the ab initio level using a minimal basis set. These polarizability and hyperpolarizability results are then used to estimate the effective (hyper)polarizabilities of the unit cell in the whole crystals and subsequently their linear and nonlinear susceptibilities. The inclusion of electron correlation effects is also discussed. [ABSTRACT FROM AUTHOR]