[Ifeffit] Independence of scattering paths?

Bruce Ravel bravel at bnl.gov
Wed Oct 1 08:53:02 CDT 2008

On Friday 26 September 2008 22:38:58 van der Veen Renske wrote:
> I have a question concerning the independence of scattering pathways. I
> fitted the EXAFS signal of a molecule consisting of 30 atoms (the
> related FEFF calculation is called calc 1). To see the effect of the
> surrounding solvent molecules in the chi(k) data, I added an O-atom in
> the coordinate list of the FEFF input file at an arbitrary distance from
> the absorbing atom (in this case Pt) and I re-ran FEFF (calc 2). As a
> check within calc 2, I generated the sum of the same scattering paths
> that were used in the fitting model of calc 1 using the therein obtained
> structural fit parameters. The new O-atom was not included in this sum.
> I expected the sum within calc 2 to be identical to the fit of calc 1
> based on the fact that the original atoms can not sense the presence of
> the new O-atom (independence of scattering paths). BUT: the resulting
> sum was significantly different from the fit!

It is a little hard to answer this question since you described it to
us rather than showing it to us with project file(s).  I have a couple
of suggestions, but without being able to examine what you did, I
cannot be sure that either is relevant.  (You can insert here my
standard diatribe on the topic of how to ask questions.)

One suggestion is to be very careful that you are including the same
set of paths in the calculation.  When you add an atom to a feff.inp
file, the indexing of the paths change in a way that may be hard to
predict.  Just blindly summing up the first N paths may not do what
you want.

The other suggestion is to check that you did not place the oxygen
atom in a position that dramatically altered the muffin tin radii.
The potantials calculation can change quite if you insert an atom at
an arbitrary loctation such that the muffin tin radii have to be very
different for some atoms.


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

 National Institute of Standards and Technology
 Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
 Building 535A
 Upton NY, 11973

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

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