[Ifeffit] help: can I use ifeffit to predict chi?

Bruce Ravel bravel at bnl.gov
Thu Nov 1 09:38:35 CDT 2007

On Tuesday 30 October 2007 21:21:59 Tai-Yen Chen wrote:
> hi,
>    Can anyone tell me whether I can use ifeffit to do this
> job?
> I want to know the % change in chi when the molecule have
> 3% change in radius?
> All I have is just the crystal struture of gamma-Fe2O3,
> which means I can only get the Feff.
> How can I introduce 3% change in redius so that I can get
> the difference of chi?
> Can anyone help me? Tanks a lot.

Hi TaiYen,

Since you are asking about using Ifeffit, I presume that you are
interested in changes in the EXAFS spectrum rather than the XANES.

There are a number of ways you might approach this problem.  Probably
the best way of doing so would be to do a proper fit to measured data
so that you can get a good measure of things like sigma^2 and e0.
Then, using the results of the fit, modify the deltaR parameters for
each path used in the fit to be 3% larger.  Then simply add up the
paths using the fitted values for sigma^2, e0, and so on.

You can also do something similar without actually doing a fit to
data.  Use Artemis or ifeffit itself to increase R for each path and
simply sum the paths without using fitted values for sigma^2 and so
on.  The easiest way of doing so would be to set deltaR for each path
equal to "0.03*reff".  That will expand the length of each path by 3%,
thanks to the magic of the special "reff" parameter in Ifeffit.

Another way of doing this (and a decent way of doing so for a XANES
calculation) would be to use Feff's rmultiplier keyword:


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

 National Institute of Standards and Technology
 Building 535A, Room M7
 c/o Brookhaven National Laboratory
 Upton NY, 11973, USA

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

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