[Ifeffit] Contribution of multiple scattering paths - basic question

[Gerrit Schmithals]

Hello,

thank you Matt and Bruce for responding to my question so comprehensively.

I have learned that it is quite possible that multiple scattering paths
do not contribute to the spectrum because of their large mean square
displacement. Is that due to the fact that the mean square displacements
are large for all atoms and for multiple scattering paths with more
atoms involved the mean square displacements are multiplied?

I now tried different variable combinations but it seems that it is
quite complicated to fit the material for higher R. That is probably due
to the fact that up to 5A FEFF calculates about 260 paths, 2/3 of them
being multiple scattering paths. I found that it is not satisfying to
exclude all paths with an amplitude <10 because there are a lot of paths
with a similar Reff and Phase shift that in sum contribute noticeably to
the spectrum.

One easy way to obtain a fit that is not too bad was to exclude all
multiple scattering paths. I then fitted with just one
mean-square-displacement and one amplitude for all paths (see
attachment). The fit looks somewhat better for higher k-weights, I am
not sure if this is a fact to worry about. For k-weight=2 the peak
positions up to the third peak are not too bad, the broad peak around 4A
is not resolved. I could probably argue that for higher R-values the
contributions of multiple-scattering paths may not be neclegted.

Cheers,
Gerrit

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