[Ifeffit] Difference File Fits

[Bruce Ravel]

On Tuesday 18 March 2003 06:56 am, Norbert Weiher wrote:

> I was just playing a bit around with some data and it sprang to my mind that 
> sometimes the idea of doing difference file spectra fits would be a nice 
> feature (like XDAP has it). This would be great to identify single 
> contributions of certain scatterers that are close to each other or somehow 
> other produce similar paths.
> 
> How about including a feature which lets you extract a "new" chi.dat file 
from 
> the original data minus some paths one has selected? This file could then be 
> further processed.

Subtracting a sum of paths from the data is pretty easy at the level
of ifeffit's math expressions.  A feature for subtracting an arbitrary
sum of paths from the data does not currently exist in Artemis, but I
can put it on the todo list.  If you choose to fit in original
k-space, the residual is basically what you are asking for.  You can
save the residual function to a file and then treat it as data in a
new fitting project.

> Finally something similar for EASE: I'm still one of those old-fashioned 
EMACS 
> folks and would like to have something like this at my hands in EASE too. 
> Bruce, what do you think about it? Would it be possible?

What you are asking for would be a little outside the scope of EASE
(though not out of the question).  Since that would be a rather low
priority chore for me, you would be much better off solving this
yourself.  Run feffit with the flags for writing the individual path
files in k-space turned on.  Then write yourself a little perl or
python script to read in the data and a number of the k.NNN files and
write out the difference spectrum.  That should be pretty easy since,
if you restrict yourself to the output files from feffit, you can be
sure that they are all on the same grid.  I bet you could solve that
in 20 lines or less of perl ;-)

B

-- 
 Bruce Ravel  ----------------------------------- ravel at phys.washington.edu
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