[Ifeffit] Debye Waller factors for Water Molecules

[Matt Newville]

Hi Paul,

A. Vogel et al (Biochemistry 43, p10379, 2004) report a Zn water (or
OH) at 2.0Ang with sigma2=0.007Ang^2 for data collected at 30K.

This is for a binuclear Zn compound, and the OH is a bridging ligand
(with the other ligands being dominated by histidine-like rings), so I
don't know how close it is to your structure.  But it might be a good
place to start.

Also, the results came from EXCURVE.  That's not to suggest that the
results are not reliable, but you might want to check carefully the
meaning of sigma2.  I'm not 100% sure of this (or what the Vogel et al
paper is reporting), but I think the EXCURVE sigma2 might be a factor
of 2 bigger than the Ifeffit sigma2.

I'd be a little wary of using an Einstein model for "water"  ligands
down to 30K.  It would be interesting to hear how well an Einstein
model compares to the DFT modeling for any ligands in a bio-molecule.

--Matt