Chemistry, University of Western Australia^{*} Chemistry, Durham University, England^{**}

â—‹Dylan Jayatilaka^{*} Andrew Whitten^{*} Paraprathim Munshi^{*} Mark Spackman^{*} Mike Turner^{**} Dima. Yufit^{**} Judith A. K. Howard^{**}

The calculation of polarisabilities and hyperpolarisabilities from X-ray diffraction data on molecular crystals has been previously been suggested, but the results have been unconvincing because the approximations made have been quite severe. In this talk we stress the importance of the off-diagonal elements of the density matrix for determining these quantities. Using the Lorentz-factor tensors, we also describe how the polarisability can be used to calculate the refractive index tensor, and the hyperpolarisability can be used to obtain the second order susceptibility tensor, important for non-linear optical (NLO) applications. Two methods are described to obtain these optical properties: one based on the Unsold approximation, another based on coupled-perturbed Hartree-Fock (CPHF) theory. Both methods are based on the use of density matrices obtained from X-ray constrained wavefunctions, but the CPHF method avoids a scaling procedure. The method is applied to a series of molecular crystals which display NLO behaviour: urea, 2-methyl-4-nitroaniline (MNA), 2-(N-nitrophenyl)-L-prolinol (PNP), N-(4-nitrophenyl)-L-prolinol (NPP).
The results will be discussed.

References: A. Whitten, D. Jayatilaka, M. A. Spackman, Journal of Chemical Physic