[Ifeffit] The most important fitting parameters

[Bruce Ravel]

Hi Bridget,

Welcome to the list!

>From what you have said, it would seem that you are on the right
track.  Without knowing what CuWO4 looks like or how it behaves, I
have a few suggestions that you might want to think about

> I started off with CuWO4 as the model compound and constrained the fitting
> parameters:
> - take all 1st oxygen shells (x6 at slightly different distances) and give
> them all amp1, dr1*reff, enot and ss1
> - take all 1st copper shells (x2) and give them both amp2, dr2*reff, enot
> and ss2
> - take all 2nd oxygen shells (x6), amp3, dr3*reff, enot, ss3
> - take all 1st tungsten shells (x6), amp4, dr4*reff, enot, ss4
>
> This gave me 13 parameters with 14.8 independent points.

I think you have not quite explained what your constraints are.  4
shells times 4 parameters is 16 variables, not 13.

It is hard to justify independent amplitude parameters for the
different shells of a crystal.  If you have any confidence at all in
the crystallography, then it is hard to justify floating the amplitude
for each shell.  More appropriate would be to constrain each shell to
have the coordination indicated by the crystallography and float a
single S02 parameter.

Similarly, it is hard to justify a priori the use of an independent
enot for each shell.  If we have faith in the theory, then you should
only need one enot parameter to line up the energy grid of the data
with the energy grid of the theory.

Those two suggestions reduce the number of parameters from 16 to 10.

You probably need to float sigma^2 independently for each shell, but
you might be able to constrain the deltaR parameters in some way.  I
don't know what the structure of CuWO4 is, but there might be some
clever constraint you could use to reduce the number of deltaR's.
That might give you enough freedom to try to determine more about the
first oxygen shell than just an overall deltaR.  Who knows?

It's a shame that you data range only goes out to 9 inverse
Angstroms. 

> (Other factors: fitted in kw=2&3, k-range 2-9, used Athena with default
> background subtraction parameters as the chi(k) from that seemed sensible.
> The data were collected at SSRL, beamline 11-2, in transmission.)

It is good that you are using the multiple-k-weight trick.  You might
consider including k-weight of 1 as well.  Using 1 and 3 emphasizes 
the high and low bits quite differently.  Here's some reading on the
topic: 
   http://www.google.com/search?q=site:millenia.cars.aps.anl.gov+correlation

As you say, getting both edges would be a boon, particularly given
that your data range is so limited.  Fortunately, it's *only* a 14
hour plane ride from New Zealand to California, so measuring more data
will be no trouble at all ;-)

HTH,
B


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

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