[Ifeffit] Generating single scattering paths in Artemis

[scalvin at slc.edu]

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 at 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?
>>
>
> --
> Carlo U. Segre -- Professor of Physics
> Associate Dean for Special Projects, Graduate College
> Illinois Institute of Technology
> Voice: 312.567.3498            Fax: 312.567.3494
> Carlo.Segre at iit.edu    http://www.iit.edu/~segre
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