[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:
http://leonardo.phys.washington.edu/feff/wiki/index.php?title=RMULTIPLIER
HTH,
B
--
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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