[Ifeffit] Ifeffit Digest, Vol 80, Issue 1

Bruce Ravel bravel at bnl.gov
Fri Oct 2 14:06:22 CDT 2009

On Friday 02 October 2009 01:15:25 pm Hashem Stietiya wrote:
> So I should not be skeptical if my fitting results return a high Debye
> Waller value for my first shell as compared with the second shell.


Carlo is of course correct to preach constant scepticism.  That said,
there are reasons that a first shell sigma^2 might be bigger than the
second shell.  I don't know what they might be in your case becasue I
have no idea what you are working on.  Your question, though, touches
upon a more general statement that is just as true when your fitting
parameters are unsurprising as when they are surprising.

If you cannot understand the physical meaning of your fitting
parameters, if you cannot explain their values in terms of something
known or suspected about the structure, then you haven't really done
anything by fitting other than to make a pretty picture.  *Every*
parameter has to be interpretable.

If there is a physical reason to expect the first shell sigma^2 to be
big, fine.  If you cannot understand that result in some sensible
manner, then your fitting model is problematic.

In any case -- surprising results or otherwise -- it is incumbant upon
the person pushing the big, green button to understand and interpret
the results of the fit.



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

 National Institute of Standards and Technology
 Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
 Building 535A
 Upton NY, 11973

 My homepage:    http://xafs.org/BruceRavel
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/

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