[Ifeffit] Fitting procedure

Matt Newville newville at cars.uchicago.edu
Sat Apr 18 12:57:18 CDT 2009

Hi Adam,

I am confused by this thread too!

As you say, the original issue involved using S02 for individual
shells of data on Cu foil.  Of course, having separate S02 for each
shell is not usually necessary to model Cu foil -- in fact I can't
think of a case where it is needed.  But  since S02 is 100% with N,
and ~95% correlated with sigma2, it is easy to have S02 vary per shell
if one allows it to happen.    Whether that's a good idea or not
depends on how you interpret the results.   Most people choose to fix
a single S02 and vary N.  Lots of people fit spectra with one S02 and
an N for each shell, so I don't think the number of free parameters is
necessarily a big issue.

In a follow-up message, Scott seemed (to me) to be more concerned
about whether it was reasonable to "fix parameters for the first
shell"  when fitting a second or higher shells, not so much with the
issue of what parameters were being refined.  There weren't explicit
details on what was fixed or what fitting ranges or parameters were
used for the higher shells.  Scott suggested (I believe) that "fixing
the first shell" and fitting the second shell was not a good idea, and
asked to hear differing views.

I find the approach of "fit the first shell, then fix those parameters
and fit the second shell" to be a reasonable way to start, so I
responded.   I would not call this approach "acceptable, but generally
not preferred" -- I would say it has actual merit.  Yes, it is often a
good idea to check a fit to the whole spectrum toward the end of an
analysis.  But in a fit to the whole spectrum, the quality of the fit
to the more subtle features (the finer structure) can often get lost
in the fit to the more dominant features of the spectra.  That is why
we subtract mu0(E), after all.  I don't see Fourier filtering as a bad
idea for analyzing higher shells.

But perhaps I am just not understanding what Scott and Shelly recommend???


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