[Ifeffit] can sigma square ever be less than zero?

Scott Calvin SCalvin at slc.edu
Mon Jun 28 15:25:56 CDT 2010

Hi Chris,

Not to be picky, but I think we have to consider the semantics of what  
you're asking very carefully.

"Can ss be negative," as in, can the physical quantity which is the  
variance in the absorber-scatterer distance be negative? No, since  
variances are the square of a real number.

"Can ss be negative," as in, can ifeffit output a negative best-fit  
value for ss? Yes, as you've seen.

"Can ss be negative," as in, can a fit with a negative best-fit value  
for ss be considered a valid fit? That's really what you're asking, I  
think, and the answer is that it could be, depending on what you are  
trying to claim. Since the uncertainty in your case is quite large,  
it's certainly possible that your fit is consistent with believable  
values. But it also means that your fit gives you very little idea of  
what ss should actually be. The twin facts that the uncertainty is  
large and that the best-fit value is very close to 0 make the fit less  

The simplest explanation in your case is that Ifeffit is finding a fit  
with an S02 and a ss that are both a bit low. Since they tend to  
correlate highly, that's not uncommon. Have you tried fitting using  
different k-weights, or, better yet, several k-weights simultaneously?

At any rate, I'd say your fit is a promising preliminary fit. As far  
as a publication-quality fit, it would be nice to get the nearest- 
neighbor ss pinned down a bit better.

--Scott Calvin
Sarah Lawrence College

On Jun 28, 2010, at 4:10 PM, Chris Patridge wrote:

> Hello all,
> I am working on W L3 edge data.  W is acting as a substitution  
> dopant in vanadium dioxide at rather low concentration.  In a past  
> mailing conversation discussing Feff6 overestimation of E0 for  
> heavier elements it was mentioned that the E0 could be past the  
> rising edge due to the white line from W data.  Well using this  
> comment I aligned data using the theory method well explained by  
> Shelly Kelly SnO2 example.  Literature suggests W approximates WO2  
> cubic structure locally instead of the VO2 unit structure.  Then  
> fitting the first oxygen coordination shell paths which are well  
> isolated from the other paths, it gives reasonable values for  
> amplitude and enot of 0.77 (0.17) and -1.14 (3.06) respectively.   
> delr is -0.067 (0.028) and then ss comes out to -0.00036 (0.00414).   
> Can ss be negative if the uncertainty brings it above 0?
> Thank you all,
> Chris Patridge
> PhD Candidate
> Department of Chemistry NSC 405
> SUNY Buffalo
> 315-529-0501
> _______________________________________________
> Ifeffit mailing list
> Ifeffit at millenia.cars.aps.anl.gov
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