[Ifeffit] Breaking down correlationships between parameters
bravel at bnl.gov
Mon Mar 23 08:27:05 CDT 2015
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.
Among the lectures at
this is the one called "Fit Evaluation and Fitting multiple structures".
Bruce Ravel ------------------------------------ bravel at bnl.gov
National Institute of Standards and Technology
Synchrotron Science Group at NSLS-II
Upton NY, 11973
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