Thanks everyone (Shelly, Matt, and Anatoly) for the info.   I thought it would be fun to learn while trying to work out a (relatively) simple system.  I have EXAFS data about a Zn site.  The Zn is bonded to three cystedines so I have the DW pretty much under control, but for the water molecules.  I will investigate the approaches mentioned here and see which one the data is most consistent with (I did have the foresight to choose a system for which I have high resolution x-ray crystallography data).

Thanks

Paul

On Jul 29, 2007, at 12:57 AM, Matt Newville wrote:

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

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