[Ifeffit] Exafs distance resolution

Kropf, Arthur Jeremy kropf at anl.gov
Tue Nov 27 11:02:57 CST 2007

> > 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
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Chemical Sciences and Engineering Division
Argonne National Laboratory
Argonne, IL 60439

Ph: 630.252.9398
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