[Ifeffit] k-weight in Artemis

Bruce Ravel bravel at anl.gov
Tue Sep 26 16:53:23 CDT 2006

On Tuesday 26 September 2006 15:45, Scott Calvin wrote:
> >And another question, I have seen an option to fit background in
> >Artemis, is it obligatory to fit the background to publish EXAFS
> >fits?
> No. This is another one of these issues where people can reasonably
> have different preferences.  Personally, I prefer not to use the
> Artemis background refinement in my published fits, partly because
> it's one more processing step to explain and justify. But I do use
> the ability of Artemis to do background refinement to test if my
> Athena background is OK: if fitted parameters correlated highly with
> Artemis background parameters, I know there's a problem, and I go
> back and look at the background subtraction process in Athena
> again. If they don't correlate highly, then the Artemis background
> wasn't doing anything except improving the visual aesthetics of the
> fit anyway, and I turn it back off.

I'd like to expand on this a bit.

At the level of Artemis, users are often concerned that the background
corefinement is somehow a cheat.  It isn't.  (Well, not any more than
anything else Artemis does -- but that's a separate story ;-) The way
to think about it is in terms of Fourier components.  Artemis uses
Feff calculations to fit the Fourier components between Rmin and Rmax
and, if the background corefinement is turned on, it uses a spline to
fit the components between 0 and Rmin.

Considered in that light, background corefinement is very handy
indeed.  If you set Rmin to be the same as Rbkg, this gives you a
chance to evaluate the correlations between your background removal
and the parameters you are actually interested in.  As Scott says,
this can give you a certain confidence in your results, or, I suppose,
encourage you to go back to Athena and do the background removal

So, the background corefinement is the use of splines to optimize
low-frequency Fourier components.  In the Autobk algorithm, we use
splines to optimize low frequency Fourier components.  It is,
therefore, reasonable to consider background corefinement as as
additional round of playing with Autobk while simultaneously trying to
figure out something about your chi(k) data.


 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

 Molecular Environmental Science Group, Building 203, Room E-165
 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007

 Argonne National Laboratory         phone and voice mail: (1) 630 252 5033
 Argonne IL 60439, USA                                fax: (1) 630 252 9793

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

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