This question comes down to the following practical problem. If one has a theoretical model developed using computational chemistry that predicts two different bond lengths within one shell, e.g. an octahedral metal center surrounded by 6 oxygen atoms and this shell is predicted to be split in three subshells for wich the bond length differs only 0.05 angstroms; and this model can be fit very well in k-space with the splitted shell, off course keeping the number of fit parameters below the nyquist criterion. Is there in such a case any physical reason not to fit the experimental data with the splitted shell , but with an averaged 6-atom shell with a larger Debye Waller factor?
My guess would be that the EXAFS could probably be fitted just as well with one distance and a slightly larger sigma2 as with 3 separate distances. But this would depend some on the data quality and it might be right at the resolution limits, so I'd recommend trying both models.
I think Matt is correct that the fits would be statistically indistinguishable. However, the paths have just enough distance difference that using a single shell will probably mess up your amplitude measurement.
Cheers,
--Matt _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Jeremy Chemical Sciences and Engineering Division Argonne National Laboratory Argonne, IL 60439 Ph: 630.252.9398 Fx: 630.252.9917 kropf@anl.gov