[Ifeffit] why ss_2 is negative?

[Bruce Ravel]

On Friday 23 February 2007 16:24, hw26 at njit.edu wrote:
> I am doing model fitting using Artemis. My sample is uranium solution with
> an ionic liquid,  1-methoxyethyl-3-methyl imidazolium tetrafluoroborate
> (MOEMIMBF4). The Fitting seems ok except that the ss_2 is negative. I don't
> know how can I fix this problem. Can you get a chance to take a look? My
> fitting is simple, only three single scattering pathes which were produced
> by Six Pack. The fitting file is attached. I really appreciate it if you
> could take a look and give some advice!

Let me ask you a couple questions that I think are closely related to
yours.  You seem to be modeling an uranyl ligand and two equatorial
ligands.  Focussing on the second path, the one with a negative
sigma^2 and the one that (I am guessing) represents either a hydration
shell or a bidentate ligand:

  1.  Is 35.996 a reasonable value for e0?
  2.  Is 2.59 Angstrom a reasonable value for an equatorial oxygen
      ligand in a uranyl moiety?

It would seem that your fit has quite a few problems beyond the
sigma^2 value.  I question whether your parameterization of these
three paths is physically defensible.

B


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