Hi Carlo, Good point...but I would note that you're choosing to ignore atoms further out radially in the FEFF calculation, so the 3-dimensional environment isn't right anyway. It would seem to me that the EXAFS differences between, e.g., tetrahedral and octahedral coordination would often be smaller than the error in leaving out the next shell, depending on the material. For example, by my count in an fcc (12-coordinated) lattice including only one shell means each scattering atom has only 3 of its 12 neighbors present in the calculation. To the extent that the FEFF calculation is impacted by such things, this is substantial. Fine for an approximate fit in many cases, but I'm not sure I see that it's worth the trouble of constructing tetrahedral (or whatever) symmetry. On the other hand, I haven't personally compared the tetrahedral to the octahedral to the single-scatterer to the full-crystal case...it might be interesting to try and see how much of a difference each makes to a fit. --Scott Calvin Sarah Lawrence College
Scott:
I think that Anatoly's posts answer this question. The calculation of the overlaps requires some kind of three-dimensional environment (or the use of the OVERLAP card which I was not familiar with and whose documentation in the FEFF manual is a bit cryptic). There are differences between a single neighbor at a specific distance and a coordination shell of the same neighbor at the same distance.
Sam's program clearly does the calculation by building a coordination sphere while Anatoly's method lets FEFF do it automagically. Apparently the results are the same.
Carlo
On Wed, 5 Jan 2005 scalvin@slc.edu wrote:
Hi all,
A question for Carlo--I guess I'm not clear on what the impact is of tetrahedral vs. octahedral or whatever if all you're doing is a single-scattering nearest-neighbor EXAFS analysis. Isn't a symmetric octahedral arrangment just the single-scattering path with a degeneracy of 6; a tetrahedral a degeneracy of 4; etc.? Why have FEFF calculate anything other than the single-scattering path?
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