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

[Matt Newville]

Hi Chris,

On Mon, Jun 28, 2010 at 3:10 PM, Chris Patridge <patridge at buffalo.edu> 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?

Well, this sigma^2 is not *very* negative, is it?  By itself, that
might be an OK place to start as an initial fit.  But I would probably
take the combined results of sigma^2 being approximately zero (never
mind it being slightly negative) and delr being far from zero (-0.07
+/- 0.03) as a suggestion that the starting model is not really the
story the data wants to tell you.

My understanding is that tungsten oxides can be challenging to model.
See Moscovici et al J Chem Phys 125, 124505 (2006), for a discussion
of what can go wrong. That paper does focus on trivalent tungsten, but
it might give useful pointers.

--Matt