[Ifeffit] "FEFF+IFEFFIT" and "cumulant expansion+ratio method" approaches

[Leandro Araujo]

Dear all,

I'm stuck on a peculiar situation and would appreciate any kind of
help available.
I need to "translate" EXAFS analysis results obtained with
IFEFFIT/FEFF into the "language" of the cumulant expansion/ratio
method approach.
More specifically, I have to relate my mean interatomic distance R
obtained with IFEFFIT to the first cumulants (mean interatomic
distances) that show up in the ratio method formalism. I'm saying the
cumulantS because the ratio method makes a distinction between the so
called "effective P(r,lambda)" and "real rho(r)" distributions of
interatomic distances, which are related by:
P(r,lambda)=rho(r)*[[exp(-2r/lambda)]/r^2] . For the second cumulant
(Debye-Waller factor or sigma^2) and higher terms, the difference
between "effective" and "real" values is not significant unless the
disorder in the sample is really big. But for the first cumulant it is
significant (at least at not very low temperatures), being the "real"
first cumulant bigger than the "effective" one by a term  like
[(2*sigma^2)/r]*[1+(r/lambda)].

My dilemma is: how my mean interatomic distance R from IFEFFIT relates
to the "effective" and "real" first cumulants? Should it be the same
as one of them? Which one? Or it doesn't correspond exactly to any of
them?

Any comments will be welcome...

Regards,
Leandro