Distributed first and second order hyperpolarizabilities: An improved calculation of nonlinear optical susceptibilities of molecular crystals

H. Reis and M. G. Papadopoulos
Institute of Organic and Pharmaceutical Chemistry, National Hellenic Research Foundation, Vasileos Constantinou 48, GR-11635 Athens, Greece

Christof Hättig
Forschungszentrum Karlsruhe, Institute of Nanotechnology. P. O. Box 3640, D-76021 Karlsruhe, Germany

J. G. Janós G. Ángyán,
Laboratoire de Chemie théorique, UMR CNRS No. 7565 Institut Nancéien de Chimie Moléculaire, Université Henri Poincaré, B. P. 239, F-54506 Vandoeuvre-lès-Nancy Cedex, France

B. W. Munn
Department of Chemistry, UMIST, Manchester M60 IQD, United Kingdom

J. Chem. Phys. 112, 6161-6172 (2000).
(Received 22 October 1999; accepted 21 December 1999)

The method of calculating distributed polarizabilities is extended to the first and second dipole hyperpolarizabilities, in order to describe more accurately the molecular response to strong and inhomogeneous external time-dependent electric fields. The dipolar response is expressed in terms of both potential related charge-density response functions and electric field related dipole-density repsonse functions. The macroscopic linear, quadratic, and cubic optical dipole susceptibilities of molecular crystals are expressed in terms of the distributed (hyper)polarizabilities. This formulation differs from previous theories using distributed dipoles in that it allows for a rigorous treatment of both local induced dipoles and charge flow between different regions of the molecule. As an example, the distributed polarizabilities and first hyperpolarizabilities of urea at the self-consistent-field level are used to calculate the linear and quadratic susceptibilties of the urea crystal. The linear susceptibility does not differ substantially from that calculated with previous less rigorous models for distributed response, but the quadratic susceptibility is about 50% of that calculated with previous models. This indicates that the present treatment of distributed reponse should give a quadratic susceptibility in good agreement with experimental data, once the effects of electronic correlation, frequency dispersion, and the permanent crystal field are taken into account.

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