[Ifeffit] Using more than one E0

[Bruce Ravel]

On Friday 28 May 2004 01:42 pm, Scott Calvin wrote:
SC> It is my secret, completely untested belief (which I am now revealing
SC> to everyone on this mailing list!), that some of the cases of
SC> "successful" fits using multiple E0's are masking problems caused by
SC> not considering a third cumulant. For those who may not know the role
SC> of this parameter, it in essence measures asymmetry in the
SC> distribution associated with a path. For example, if a pair of atoms
SC> are more likely to separated by a distance considerably larger than
SC> the mean separation than by a distance considerably smaller than the
SC> mean separation, then the third cumulant is positive. (The
SC> mathematical definition is that the third cumulant is the mean cube
SC> of the difference from the mean, in the same sense that sigma2 is the
SC> mean square of the difference from the mean.)
SC>
SC> In most cases, the third cumulant is small. Nevertheless, if it were
SC> 0 in all cases, then materials would not show any expansion with
SC> temperature!

This is a really good point.  A non-zero C3 is often much easier to
justify physically than a second e0 parameter.

As I recall, Wojciech is working on some kind of solvated complex.
Without the rigidity of a crystal form, I think that it is quite
reasonable to expect that something solvated would have a measurable
C3.

B


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