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 -- Bruce Ravel ----------------------------------- ravel@phys.washington.edu Code 6134, Building 3, Room 405 Naval Research Laboratory phone: (1) 202 767 2268 Washington DC 20375, USA fax: (1) 202 767 4642 NRL Synchrotron Radiation Consortium (NRL-SRC) Beamlines X11a, X11b, X23b National Synchrotron Light Source Brookhaven National Laboratory, Upton, NY 11973 My homepage: http://feff.phys.washington.edu/~ravel EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/