[Ifeffit] Questions on Correlated Debye in FEFF6

[Bruce Ravel]

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

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