[Ifeffit] Constraining values in Artemis fit

Bruce Ravel bravel at bnl.gov
Thu Aug 23 11:13:11 CDT 2018


On 08/23/2018 10:55 AM, Thomas, Andrew (AGW) wrote:
> Hello,
> 
> 
> Does anyone have any suggestions for breaking the correlation between 
> the coordination number and sigma^2? Specifically, is there a way to set 
> a maximum or minimum value for parameters in a fit?

Matt is, of course, correct when he says that this correlation is 
inherent to EXAFS analysis and cannot be "broken".  I would like to 
suggest a broader way of thinking about this sort of issue.

When you have a very well behaved analysis problem -- that is, something 
like my teaching example of FeS2, something for which the structure is 
pretty well known -- then your EXAFS analysis will yield defensible 
values for CN and sigma^2 without too much effort.  They will still be 
correlated, but sensible numbers will tend to just fall out.

In a harder problem -- y'know something you are doing actual research on 
-- you often run into the situation where these correlations preclude a 
"fall right out" analysis.  It is tempting to assert that the CN must be 
SOMETHING and the ss must be SOMETHING.  That's true in a sense, but you 
have made a real measurement and are doing a real analysis with real 
measurement uncertainties.  And you have to respect that.

When you are running into trouble in your EXAFS analysis -- big 
uncertainties, indefensible values, that sort of thing -- that is 
usually the program trying to tell you something about your analysis. 
Usually, that would mean that you want to know something that the data 
do not support (or do not support beyond some level of precision or 
accuracy).  Or it might mean that your fitting model is not realized in 
the data -- that is, your model is missing some important feature and 
the bad fit results are the result of missing that important feature.

To summarize, I want to encourage you not to assert that you need to 
find a way to break the correlations.  i want to encourage you think 
about what the wonky fit results are trying to tell you about your data 
or your fitting model.


That was kind of rambling, I admit.  Hopefully it was helpful 
nonetheless....

B


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

  National Institute of Standards and Technology
  Synchrotron Science Group at NSLS-II
  Lead Beamline Scientist, 06BM (BMM)
  Building 743, Room 114
  Upton NY, 11973

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