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Title: Effective molecular polarizabilities and crystal refractive indices estimated from x-ray diffraction data
Contributor(s): Whitten, Andrew (author); Jayatilaka, D (author); Spackman, MA (author)
Publication Date: 2006
DOI: 10.1063/1.2364897
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Abstract: Although it was proposed some time ago that (hyper)polarizabilities might be estimated from the results of x-ray charge density refinements, early results were unconvincing. In this work we show that the one particle density obtained from the usual multipole refinement model does not contain sufficient information to determine these response properties and instead pursue the "constrained wave function" approach of fitting to x-ray structure factors. Simplified sum-over-states expressions are derived for determining the dipole polarizability from these wave functions, and these clearly show that the earlier work ignored important two-electron expectation values for the dipole polarizability, and two- and three-electron terms for β, etc. Correction factors for the simplified sum-over-states polarizability tensors from the constrained wave function are obtained by calibration against coupled Hartree-Fock ab initio results to yield in-crystal effective polarizability tensors. Results obtained for benzene, urea, and 2-methyl-4-nitroaniline demonstrate that the effective molecular polarizabilities clearly include the effects of intermolecular interactions and electron correlation, especially for urea where the effects on the polarizability are known to be quite large. We also carefully consider the way in which the linear bulk susceptibility, Χ⁽¹⁾, and refractive indices are determined from the x-ray fitted polarizabilities, employing three models based on a rigorous treatment of the local field. Incorrect results are obtained for the sort of molecules that are of interest in nonlinear optical applications if the molecules are approximated by single point dipoles. In contrast, the use of Lorentz-factor tensors averaged over several sites yields excellent results, with refractive indices obtained using this model in remarkably good agreement with optical measurements extrapolated to zero frequency.
Publication Type: Journal Article
Source of Publication: Journal of Chemical Physics, 125(17), p. 174505.1-174505.14
Publisher: American Institute of Physics
Place of Publication: United States
ISSN: 0021-9606
Field of Research (FOR): 020404 Electronic and Magnetic Properties of Condensed Matter; Superconductivity
Peer Reviewed: Yes
HERDC Category Description: C1 Refereed Article in a Scholarly Journal
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