[Ifeffit] Breaking down correlationships between parameters

[Bruce Ravel]

On 03/23/2015 08:54 AM, Rana, Jatinkumar Kantilal wrote:
> The term N*S02 is fitted for each path of the FEFF calculation. So my
> question is, even if we know N with a great certainty for some path,
> how can we vary both N and S02 for other paths ? or Did I understand
> it wrong ?

At no point in your emails did you say that you are using Artemis to do 
your fitting.  The comment I am about to make may not, therefore, be 
relevant to you.

In Ifeffit and Larch -- and therefore in Artemis -- we DO NOT fit N*SO2. 
  Fits in Ifeffit, Larch, and Artemis use a set of user-defined 
parameters as the variables of the fit.  As part of the fitting model, 
the user relates the variables of the fit to the parameters of the EXAFS 
equation using math expression.

This benefit of the generality of the fitting model allows the user to 
easily encode prior knowledge into a fit.  The cost is that the 
parameters have to be interpreted in some kind of physical context.

To answer you question specifically, Artemis allows you to set or float 
parameters for N and S02 and to define the amplitude term of the EXAFS 
equation for each path in any way that you see fit.  In that way, you 
can implement all the suggestions that Matt, Scott, and Chris have made.

I discuss this in some detail starting at page 35 of this presentation.

   https://speakerdeck.com/bruceravel/advanced-topics-in-exafs-analysis

Among the lectures at 
http://www.diamond.ac.uk/Beamlines/Spectroscopy/Techniques/XAS.html, 
this is the one called "Fit Evaluation and Fitting multiple structures".

HTH,
B


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

  National Institute of Standards and Technology
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