[Ifeffit] Debye Waller factors for Water Molecules

[Matt Newville]

Hi Paul,

I'm not sure that I'd expect a "zinc-water" ligand in a bio
macro-molecule to act like zinc in an aqueous solution.  The zinc will
really be bound to that one "water" in the molecule, which is pretty
different than a zinc ion in solution, where exchange of water will
happen very frequently (although perhaps not so much at 20K, the low
temperature might just emphasize the point that an aqueous solution
isn't that good a model).  Given that the "zinc-water" bond is
somewhat different than a "zinc-water" in solution, and that zinc also
binds to cysteine,  I wouldn't be surprised by a very different
sigma2.  For example, static disorder (from bond-to-bond variations in
bond length) is probably very small for these molecules.

It would be interesting to find out how transferable metal-"water"
bond lengths and sigma2 are from solutions to bio-molecules.  I'd
expect that a literature search would be enough to tell you.

--Matt