Ummmm.... I think that Dave was actually refering to the peak fitting dialog in Athena. I can see the use of such a thing. A Gaussian, say, fitting a peak in the near edge should certainly never be -1eV wide or 20 eV wide. Somehow putting boundaries on those parameters would be of value.
Oops! He did say Athena...it's not good for me to get my Greek gods mixed up. Aside from the fact that I was supposedly a Classics minor, some of them have thunderbolts...
I concur that defending the wieghting function is the dicey part. But some sorts of restraints seem like an outstanding idea to me. Matt's example of bond valence sums as a restraint is one such.
Absolutely. In fact, they were a large part of my motivation for upgrading from feffit to ifeffit. For example, I sometimes use restraints when I expect a parameter to be the same as for a standard I have measured. If a fit to the standard yields an S02 of 0.88 +/- 0.07, it seems reasonable to restrain the sample to have an S02 near 0.88 using the uncertainty of 0.07 to guide my assignment of the weighting. Also, fits with restraints are very useful to me as a diagnostic. If a fit is insisting on an S02 of 2.63, for example, I'll try restraining the S02 to 0.90 (weighted in such a way that +/- 0.20 is not too heavily penalized). If the fit then happily chooses 0.87 or something like that, I know I'm dealing with a true "false minimum." If, on the other hand, the fit pulls the S02 as high as it can given the penalty (say to 1.50) then I know it's some other kind of problem. --Scott Calvin Sarah Lawrence College