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