[Ifeffit] Exafs distance resolution

[Bruce Ravel]

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 at 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/