Hello! 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! Am I overlooking something here? Aren't the calculated backscatter amplitudes and phases independent from each other? I am using Artemis version 0.8.007, Windows XP, Ifeffit 1.2.9. Thank you! Best regards, Renske Renske M. van der Veen PhD student EPFL / PSI Paul Scherrer Institut Swiss Light Source WSLA/210 5232 Villigen PSI Switzerland Phone: +41 56 310 5362 Fax: +41 56 310 3151 E-mail: renske.vanderveen@psi.ch Web: http://lsu.epfl.ch http://femto.web.psi.ch
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@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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van der Veen Renske