[Ifeffit] determining reasonable fitting parameters

[Bruce Ravel]

On 07/20/2016 06:59 PM, Neil M Schweitzer wrote:
> In terms of the DWF’s in general, what value is considered too high? I
> know the DWF’s have a component that relates to temperature induced
> disorder in the scattering shell and a component that relates to
> physical disorder in the scattering shell, but what value would be
> considered too big for a sample measured at room temperature. I have
> seen values as high as 0.03 and 0.04 in presentations (sorry, no
> references) but these seems too large to me. At some point, if the
> sample is disordered enough, it seems like EXAFS is no longer an
> appropriate characterization tool to use. What value of DWF would that
> represent (for a sample measured at room temperature)?

This one is a bit easier than your first question, so I'll take a stab 
as I am drinking my morning coffee.

As you have certainly noticed, defensible sigma^2 values tend to be 
0.00something.  Values for sigma^2 that are 0.0something tend to be very 
unrobust parameters in the sense that they cause so much attenuation 
that they more-or-less serve to remove the scattering path from the fit. 
  It is likely that the uncertainty will be similarly large in a fit 
like that.

One point I often try to get across when answering questions here is 
that often things that are presented as problems are actually useful 
information.  That is, a sigma^2 of 0.0something is trying to tell you 
that the Fourier components represented by the path are not represented 
(or represented very weakly) in the actual data.  A big sigma^2 is 
Artemis' way of suggesting one of your assumptions about the structural 
model might not be quite right.

In that case, it is likely that the fit will not change -- and might 
improve by virtue of reducing the count of guess parameters by one -- if 
you remove the path and remove the parameter.

To look at it another way: a result of "it's too disordered to measure" 
*is* a result.  It may not be quantitative.  It may not be what the boss 
is looking for.  But it's honest and it is a result.

HTH,
B


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

  National Institute of Standards and Technology
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