On Thursday 22 January 2004 11:57 am, Matt Newville wrote:
I agree with most of your answer to Dan about setting constraints
and restraints, but I think I'd fail the 'quiz' you gave:
A word of caution. Restraints can be dangerous. Here is a quiz: Why is this definition of the restraint a bad idea?
set scale = -100 restrain amp = abs(amp_guess - amp_target) * scale
where you had
guess amp_guess = 0.85 set amp_target = 0.85 set scale = 100 restrain amp = abs(amp_guess - amp_target) * scale
as a recommended way to set just such a constraint. I believe there's no difference. Am I missing something?
No... probably I am missing something. It seems to me that if you use the absolute value to force the difference to be a positive number, then set the scale to a negative number, that would *reward* a large difference rather than *penalize* it. But perhaps I am confused in how the restraints are added to chi-square. I am under the impression that restraints are added as written to chi-square. This seems consistent with the language in section 7.9 of the reference manual. However, if the restraint is squared before adding to chi-square then what I said about the negative value of the scaling term is clearly wrong. I should point out that I have not actually looked at the source code... B -- Bruce Ravel ----------------------------------- ravel@phys.washington.edu Code 6134, Building 3, Room 222 Naval Research Laboratory phone: (1) 202 767 5947 Washington DC 20375, USA fax: (1) 202 767 1697 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/