[Ifeffit] How to select important MS paths in wurtzite?

[Scott Calvin]

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...