On Saturday 19 June 2004 01:24 pm, Scott Calvin wrote:
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, That's a very nice explanation of the utility of restraints. I suspect that many out there in mailing-list-land will appreciate your comments quite a bit. Perhaps you could discuss more explicitly on how the error bar guides your choice of weight, maybe even with an example... 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/