[Ifeffit] correlated Einstein model and Debye model

[Bruce Ravel]

On Wednesday 01 February 2006 13:14, Renato Canha Ambrosio wrote:
> As described in page 80 of the EXAFS ANALYSIS WITH FEFF AND FEFFIT - PART
> 2: COMMENTARY by Bruce Ravel the eins function is a function evaluated in
> the context of the current path. So the reduced mass is calculated with the
> mass of the atoms in the path. Its ok but I have several scans at different
> temperatures so I would like to refine not only the Theta (Einstein
> temperature) but also the reduced mass and perhaps the static disorder. Can
> I do it in the context of the path or I have to perform a non - xafs
> refinement with the file containing the temperatures and sigma^2. I make
> the same question for the Debye correlated model.

That's a very interesting question.  I have a few suggestions of
things to think about.

1.  Considering static disorder in the context of a fit with ifeffit
    and the einstein model is pretty straight forward.  You can use a
    math expression like

         eins(temp, thetaE) + sig_static

    then use thetaE and sig_static as quess parameters.  Those two
    terms might prove to be rather highly correlated, but if you do a
    multiple data set fit with enough temperatures, you'd have a good
    chance of breaking the correlations.

2.  You could just float a single parameter that is used for all parts
    of the sigma^2, then analyze that parameter as a function of
    temperature once the fits at all temperatures are complete.  I
    think that is what you mean by a "non-xafs refinement".  That has
    the advantage of being flexible and unburdened by the requirements
    of doing the XAFS fit.

3.  You could do the einstein model but not use Ifeffit's eins
    function.  The formula for the eins function is given in
    Sevillano, Meuth, Rehr, Physical Review B 20:12 (1979)
    pp. 4908-4911.  You could make math expressions in Ifeffit that
    encode that formula then use any part of it as a guess parameter.
    This might be a bit complicated and error-prone to implement, but
    is certainly possible.  It has the advantage of being very general
    and you get to evaluate correlations between the terms you are
    actually interested in and the other terms in the fit.

Hope that helps,
B

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

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