On Tuesday 27 November 2007 12:02:57 Kropf, Arthur Jeremy wrote:
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.
I agree, in substance at least, if not in vocabulary. At some point the split in distances will be large enough that a Gaussian is just not a good description of the actual partial pair radial distribution function. The Gaussian distribution that you use in that fit will have some centroid (i.e. bond length), some width (i.e. sigma^2), and some amplitude (i.e. coordination). Those three numbers will be quite valid in a statistical sense, but may be quite difficult to interpret in the context of the actual bi- or multi-modal distribution. I suspect "difficult to interpret" is what Jeremy means by "messed up". B -- Bruce Ravel ----------------------------------- bravel@bnl.gov National Institute of Standards and Technology Synchrotron Measurements Group, Beamlines X23A2, X24A, U7A Building 535A, Room M7 Brookhaven National Laboratory Upton NY, 11973, USA My homepage: http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/