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 ------------------------------------ bra...@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
Homepage: http://bruceravel.github.io/home/
Beamline: https://www.bnl.gov/ps/beamlines/beamline.php?r=6-BM
Software: https://github.com/bruceravel
Demeter: http://bruceravel.github.io/demeter/
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