[Ifeffit] Re: fitting with artemis
Bruce Ravel
ravel at phys.washington.edu
Thu May 22 08:33:03 CDT 2003
Gleb,
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:
http://cars9.uchicago.edu/ifeffit/refman/node55.html
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.
HTH,
B
--
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)
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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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