[Ifeffit] Questions on Correlated Debye in FEFF6
Bruce Ravel
bravel at bnl.gov
Fri Jun 19 07:48:10 CDT 2015
Doran,
You mentioned not finding what you were looking for in the code, so
most of my answers are in the form of links to the relevant bits of
code.
> I have just recently begun learning about EXAFS and running EXAFS
> simulations using FEFF6. I have a few very basic questions about the
> underlying calculations for the correlated Debye model implemented in
> FEFF6 (called by "DEBYE" keyword). For the questions below, I am
> assuming I input a single crystallographic structure and want the
> correlated Debye model to simulate the influence of a thermal
> distribution on the EXAFS spectrum. I would appreciate any insight you
> can give me into the questions below. I would also welcome any and all
> references for the original papers where that is appropriate.
This is the reference for the correlated Debye model used in Feff,
Ifeffit, and Larch: http://dx.doi.org/10.1103/PhysRevB.20.4908
> 1. Are the Debye-Waller factors calculated for each path individually?
> (It seems like they should be since the paths will have different levels
> of influence from the thermal distribution of atomic positions)
Yes. From the Ifeffit manual:
http://cars.uchicago.edu/~ifeffit/refman/node19.html
From the Larch manual:
http://cars.uchicago.edu/xraylarch/xafs/feffpaths.html#models-for-calculating
> 2. Assuming the DW factors are calculated path-by-path, is the magnitude
> of the DW factor determined by assuming the total path length R is the
> appropriate length to use for the correlation term in the Debye spectral
> density? It seems like it would not be reasonable to treat all paths of
> the same R as having the same Debye-Waller factor since a single
> scattering path and multiple scattering paths are perturbed by a
> different set of relative atomic motion that are likely to have
> different correlations. I couldn’t locate a clear statement about how
> this calculations is actually done within the code.
In Ifeffit:
https://github.com/newville/ifeffit/blob/master/src/lib/sigms.f
In Larch:
https://github.com/xraypy/xraylarch/blob/master/plugins/xafs/sigma2_models.py
The CDM is calculated on a path-by-path basis. R matters.
> 3. Is the C1 shift that results from the vibrational motion normal to
> the bond axis along a path incorporated in the calculation? (Presumably
> using \Delta C1 = sigma_perp^2/(2<r>)) And is this formula still
> appropriate in multiple-scattering paths?
In Ifeffit:
https://github.com/newville/ifeffit/blob/master/src/lib/chipth.f#L103
In Larch:
https://github.com/xraypy/xraylarch/blob/master/plugins/xafs/feffdat.py#L377
> 4. Assuming the C1 shift is incorporated, does the correlated Debye
> model assume that the perpendicular and parallel displacements have the
> same spectral density?
I think the answer is yes, but I am not sure I understand the
question.
HTH,
B
--
Bruce Ravel ------------------------------------ bravel at bnl.gov
National Institute of Standards and Technology
Synchrotron Science Group at NSLS-II
Building 535A
Upton NY, 11973
Homepage: http://bruceravel.github.io/home/
Software: https://github.com/bruceravel
Demeter: http://bruceravel.github.io/demeter/
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