[Ifeffit] Independence of scattering paths?

[van der Veen Renske]

Dear Bruce and Jakob

Thanks for your replies. I realize that I should have been a bit more
precise in where I actually put this extra atom: it is placed outside
the molecule at an axial position of the absorbing Pt atom, representing
an ethanol solvent molecule that coordinates. It is not placed between
any other atoms that are included in the fit/sum, so the MT potentials
should not be disturbed. 

Thanks to the nice presentation of scattering paths in Artemis, I am
sure that I chose the same scattering paths involving the same atoms for
both sums.

I attached an Artemis project to make things more clear.

Thanks a lot & best wishes,
Renske




> 
> 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.
> 
> B
> 
> 
> --
>  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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