Hi Francisco,
Following up on Matt's comment, a FEFF calculation with default
option assigns sigma2 to be 0, and that probably won't result in
relative contributions between SS and MS that look much like what
happens in the real material.
A better strategy would be to read the calculation into Artemis, as
Matt suggests, and use a Debye model for the sigma2 for the paths,
trying a few different Debye temperatures to get a sense of how it
affects the relative contributions. While the Debye model is not an
appropriate model for quantitatively fitting most materials, at least
it takes a stab at how sigma2 might depend on R, which will help with
understanding how important the MS paths are to chi(k).
--Scott Calvin
Sarah Lawrence College
On Jun 17, 2011, at 7:21 PM, Matt Newville wrote:
Hi Francisco,
On Fri, Jun 17, 2011 at 7:34 PM, Francisco Garcia
<garcia.ff....@gmail.com> wrote:
Dear users,
I would like to quantify the single scattering (SS) and multiple
scattering (MS) contributions to the EXAFS spectra over a range of k
values. I adopted the following approach and I would like ask
experienced users if my approach is sound:
(1) Run a regular EXAFS (default NLEG=8); I assume this includes all
possible scattering paths. Call the chi(k) data chi.dat
(2) Run another EXAFS but this time I set NLEG=2. I assume this is
for
all SS contributions. Call the chi(k) data chi_ss.dat
(3) To obtain the MS contribution, I subtracted chi_ss.dat from
chi.dat
If my approach is faulty, can you tell me how to remedy it?
Thank you.
That approach would work under the assumption that the atomic
coordinates in the feff.inp file fully described the distribution of
atoms in the system. If, on the other hand, the feff.inp had an
idealized structure, then the approach would model the MS
contributions for that structure. For example, a feff.inp file often
assumes no static or thermal disorder in the system. In that case,
the MS contributions would have no disorder terms (eg, sigma^2)
applied to them.
An equivalent approach that may be somewhat simpler would be to use
Artemis to run the Feff calculation with NLEG=8 (4 is probably good
enough below 6Ang in most structures), read in all the paths, and sum
the paths with NLEG>2.
--Matt
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