[Ifeffit] Fit in R and k space

[Bruce Ravel]

On Tuesday 12 July 2005 04:05, Stefano Ciurli wrote:
> >I'm not sure what you're suggesting--a real simultaneous fit, which
> > somehow weights the fit in R-space and k-space to arrive at a single set
> > of parameters? Or just two fits in series so that you only hit one
> > button?
>
> first of all let me tell you that the idea of running the fit in
> R-space somehow disturbs my simple mind, considering that the R-space
> depends on the processing parameters (window type and width, weight
> etc). So, I would assume that a fit in k-space would be auspicable.
> Then, I discovered that EXCURV, the competing program at least for
> biological XAS, performs the fit BOTH in R-space and in k-space, and
> I am pretty sure that the parameters it gets are slightly different.
>
> then, yes, I was thinking of two different fits hitting the same
> button (let's say in Diana - or Artemis - instead of having just the
> option of running the fit either in R- or in k-space, one could have
> an additional button telling the program to run both fits)

My US$0.02 worth:

Conceptually, corefining in k and R space is similar to corefining multiple
k-weights.  That is, it is corefining two different perspectives on the same
data set, thus adding more points to the evaluation of chi-square without
changing the information content of the data.

The multiple k weight fit provides a clear benefit.  The different
k-weightings affect different parameters differently.  k^3 enhances, in a
sense, the sensitivity of the fit to delta_r relative to delta_e0 while k^1
does the reverse.  Doing a 1,3 fit is a sort of compromise in hopes of
partially disentangling the correlations between those parameters.

I don't see the similar advantage to doing a k,R fit.  That is not to say
there isn't one, its just says that the merit of doing so has not yet
percolated into my tiny brain.  I just don't see how a k,R fit will serve to
disambiguate some part of the fit that is ambiguous in a k-only or R-only fit.

B

P.S. I would be surprised if the choice of k-space window function has an
effect on the fitting parameters outside their uncertainties so long as a
modest value for dk is used.


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