Hi Mauro, I don't have an answer to your question, but I will ask another that I've often wondered about: why do you prefer keeping the number of multiple scattering paths small? Do you actually have enough free parameters and a stable enough fit so that you're assigning individual sigma2's to each path, including the multiple-scattering paths? If so, I understand. But if not, what's the benefit to keeping the number of paths artificially small? Personally, I've usually approached this kind of question in an empirical way. I'm generally using heuristic constraints on my sigma2's anyway. For example, I might try constraining all direct scattering paths to oxygen beyond the nearest neighbor to the same value of sigma2. By trial and error, I can determine if the fit is significantly improved by splitting these up, or significantly degraded if I, for example, constrain the nearest-neighbor and distant oxygens to the same value (usually the answer is yes to the latter!). Given that kind of approach, I've developed some schemes for constraining multiple-scattering paths in oxides in terms of the values for direct scattering paths that seem to work fairly well. Since these schemes add no additional free parameters, I just keep adding groups of multiple-scattering paths of lesser and lesser importance until they stop having an effect on the fit. Incidentally, I believe my constraint schemes are justified because in most cases I've tried if I take a promising fit without multiple-scattering paths included and then add multiple scattering paths constrained in that fashion, the statistical quality of the fit improves. This suggests that while the constraint schemes are doubtless imperfect, they are better than leaving the paths off altogether, and they cost me no free parameters. --Scott Calvin Sarah Lawrence College At 10:47 AM 5/15/2007, you wrote:
Dear all,
Do you know if in literature there are studies on the important multiple scattering (MS) paths in fitting wurtzite XAFS spectra? I mean which are the important paths to properly fit a wurtzite structure up to 6 A in a elegant way (<20 paths)?
I'm asking this question thinking at old works by Filipponi et al. on the fcc and bcc structures explaining very well the importance of some specific triangular and collinear paths to fit the EXAFS spectrum...