[Ifeffit] SS and MS contributions to EXAFS

[Matt Newville]

Hi Francisco,

On Fri, Jun 17, 2011 at 7:34 PM, Francisco Garcia
<garcia.ff.000 at 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