[Ifeffit] Contribution of multiple scattering paths - basic question

[Gerrit Schmithals]

Hello,

I want to fit a cobalt-porphyrin for which I have an atom.inp-file and
EXAFS experimental data at Co K-edge . I started with a simple sum of
paths for all paths with a distance below 5 A and an amplitude above 10
(see attachment). If one compares experiment (black curve) with this
first fit (red curve) the first shell peaks overlap quite nicely but not
the second and third (fourier transforms of the single paths are also
drawn as blue and red curves). Then I excluded four paths (the red ones)
and did a second sum of path. Now the other shell peaks overlap as well
(green curve). The four exluded paths are multiple scattering paths via
one nitrogen and one carbon atom. I conclude that the extinction of the
second shell peak in the first simulation is due to the fact that these
paths have a different phase shift than the other paths with a similar
frequency.

Do I make a simple error (adjustments of the fitting parameters
necessary?) or does that tell me something about the system? I
appreciate any help.

Regards,
Gerrit
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