AB-INITIO SINGLE- AND MULTIPLE-SCATTERING EXAFS DEBYE-WALLER FACTORS - RAMAN AND INFRARED DATA

The extended x-ray-absorption fine structure (EXAFS) Debye-Waller fact or is an essential term appearing in the EXAFS equation that accounts for the molecular structural and thermal disorder of a sample. Single- and multiple-scattering Debye-Waller factors must be known accurately to obtain quantitative agreement between theory and experiment. Since the total number of fitting parameters that can be varied is limited in general, data cannot support fitting of all relevant multiple-scatt ering Debye-Waller factors. Calculation of the Debye-Waller factors is typically done using the correlated Debye approximation, where a sing le parameter (Debye temperature) is varied. However, this procedure ca nnot account in general for Debye-Waller factors in materials with het erogeneous bond strengths, such as biomolecules. As an alternative pro cedure in this work, we calculate them ab initio directly from the kno wn or hypothetical three-dimensional structure. In this paper we inves tigate the adequacy of various computational approaches for calculatin g vibrational structure within small molecules. Detailed EXAFS results will be presented in a subsequent paper. Analytical expressions are d erived for multiple scattering Debye-Waller factors, based on the plan e wave approximation. Semiempirical Hamiltonians and the ab initio den sity functional method are used to calculate the normal mode eigenfreq uencies and eigenvectors. These data are used to calculate all single- and multiple-scattering Debye-Waller factors up to a four atom cluste r. These ab initio Debye-Waller factors are compared to those calculat ed from experimental infrared and Raman frequencies. As an example com parison with experimental EXAFS data from GeCl4, GeH3Cl gases are also reported. Good agreement,is observed for all cases tested. [S0163-182 9(98)05430-7].