[Ifeffit] Fwd: average first shell position and width

[Bruce Ravel]

On Thursday 28 June 2007, Matt Newville wrote:
> > Also, when obtaining the XAFS fits to the data, I was using: Theory
> > --->  first shell fit.
> > In this, I was giving r_eff for Se-Se bond to be 2.32 A and that for
> > Ge-Se to be 2.368 A. Through the XAFS refinement, I obtained r, dr as
> > well as r_eff.
> > which of these should be considered as the actual bond length (r or
> > r_eff)? and how are these related to the average position that I get from
> > fitting the first PDF peak in this system?
>
> The bond length is r_eff + dr.  That's the centroid of the
> distribution that best matches the data.
>
> How that relates to the g(R) measured by PDF is much a more
> interesting question.  At some level, it will depend on how you model
> the g(R) to best match the PDF data.


Well.... it has as much to do with how you model the distribution in
EXAFS as in PDF.  Using the Artemis' quick first shell utility with no
further modification conflates structural and thermal disorder.  How
those two terms get mixed together will have significant bearing on
how you might relate the parameters of the distributions in EXAFS and
PDF.

If you are using a very simple-mided model to interpret the EXAFS (
Artemis' quick first shell theory utility certainly provides you withj
a *very* simple mided model) but a rather sophisticated model to
interpret the PDF, then I suspect it will be very hard indeed to
relate the parameters of the distributions in a physically meaningful
way. 

B



-- 
 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

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