[Ifeffit] Re: fitting with artemis

Bruce Ravel ravel at phys.washington.edu
Thu May 22 08:33:03 CDT 2003


I trust you will not mind if I CC my answer to your question to the
Ifeffit mailinglist.  It is, I think, of sufficiently general interest.

> How can I constrain a guess parameter to some limited range of
> values during a fit?  For example during a fit I want my DW factor
> be varied but in the limited range, e.g. from 0.002 to 0.01?  In the
> ifeffit doc I hav'nt found a math expression for confining a guess
> parameter.

There are two possible answers.

1. To constrain a value as you wish, this will work:

     guess   x   0.005
     set     y   max(0.002, min(x, 0.01))

   That idiom will require the value never to be smaller than 0.002
   and never larger than 0.01.  A caveat: If, without this constraint,
   the fit wants that parameter to become too large or too small, than
   the likely result of this constraint is that the value will be
   pinned at one end or the other.  The min/max idiom often does not
   behave as you'd like.

2. The other option is to use a restraint as described in the Ifeffit
   reference manual:
   Artemis does not yet have an interface to setting and using
   restraints, but you could use Artemis to write and save an ifeffit
   script, add the restraint by hand, and then run the ifeffit script
   from the command line.  The ability to add restraints to a fit will
   soon be handled well by Artemis, though.


 Bruce Ravel  ----------------------------------- ravel at 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, X24c, U4b
 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/

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