[Ifeffit] Athena: problems with LCF

[Bruce Ravel]

Nina,

I see that no one has responded to this email, which arrived while I
was on travel.

As I alluded to in my email from earlier today, yours was the sort of
question that is very hard to answer.  While I am sure that your
explanation was clear to *you*, it is less so for the rest of us.

If you can document your problem -- preferably in a way that one can
reproduce on their own computer -- you will be much more likely to get
a useful answer.

As for the algorithm, it is as simple as the name implies.  The
standards are added up according to their weights and compared to the
unknown.  The weights are the variables in the fit.

If you open the "Ifeffit buffer" from the Edit menu, you can watch the
ifeffit commands as they are being sent to the Ifeffit library.  This
will show you (albeit in a rather verbose manner) how the data are
being prapred for the fit.  The call to the minimize() function then
sends the fit off to a standard Levenberg-Marquardt non-linear,
least-squares minimizer.  You can also examine the Athena source code
at http://cars9.uchicago.edu/svn/horae/trunk/athena_parts/lcf.pl but
you will find that the encoding of the fit is interspersed in a rather
confusing way with the graphical layout.

B

On Wednesday, August 03, 2011 08:53:19 am Nina Siebers wrote:
> Dear All,
> 
> I acquired Cd L3-edge spectra of some binary and ternary mixtures in
> varying proportions and for the individual components. Then, I tried
> to fit the reference spectra to the spectra of the mixtures using
> linear combination fitting of Athena to get their abundance. However,
> results were disappointing despite all spectra were carefully energy
> calibrated and normalized, so I decided to create simple mathematical
> ternary mixtures by summing up the spectra of the individual reference
> spectra (edge step normalized) and tried the fitting again to exclude
> mixing-failures and check sensitivity of LCF with the idealized
> spectra. Unfortunately, LCF was also not able to reliable deconvolute
> these spectra into the individual reference spectra.
> Does anybody have an explanation for that?
> It would be nice if somebody could give me information about the
> mathematical fitting algorithm implemented in Athena.
> 
> Thanks a lot!
> Wishes,
> Nina
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-- 

 Bruce Ravel  ------------------------------------ bravel at bnl.gov

 National Institute of Standards and Technology
 Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
 Building 535A
 Upton NY, 11973

 My homepage:    http://xafs.org/BruceRavel
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/