[Ifeffit] multiple k-weight fitting

Bruce Ravel ravel at phys.washington.edu
Mon Nov 4 08:21:31 CST 2002

Hi folks:

My US$0.02 on the topic of multiple k-weight fits:

There is actually value to both of the k-weighting tricks mentioned by
Paul and Matt.  While I agree 100% that Ifeffit and/or Artemis should
include some slight of hand to manage corefinement of two or more
k-weights of the same data, it is not obvious to me that a
multi-k-weight co-refinement will necesarily yield the same result as
the constrained parameter space mapping that Paul described.  For many
situations I suspect that they will, but it is easy to imagine a
chi-square manifold that has many false minima deep enough to trap
even a multi-k-weight co-refinement.

So I think that there is still value to the procedure that Paul
decribed, even in the presence of technological advancements by Matt.
Indeed, I think that parameter mapping is so important that something
like the procedure described in Biochemistry 35 (1996) pp. 9014-9023
(and in other articles by that crowd) is on my list of things to do in
Artemis, albeit down on the list after some other stuff and not likely
to appear in the next few months.  ;-(

As for the mechanism of how the multi-k-weight thing should be
implemented -- well, I am not sold one way or the other on whether it
should be a native part of the ifeffit() command or whether it should
be something that Artemis in her madness should deal with.  On one
hand it would probably be easier for me to have Artemis generate text
to do multi-k-weight as a multiple data set fit that it would be for
Matt to modify the core.  On the other hand, doing a multi-k-weight
fit **DOES NOT** increase th amount of infomration available for the
fit.  Having it be a part of the ifeffit() command would guarantee
that a silly user would not convince himself that the information was
doubled simply by doing a k1/k3 fit.  That is probably an important
enough point that it suggests that ifeffit() is a better place for

As Matt said, keep your eyes open for a new release of ifeffit and of
my codes later this week.  There have been some huge improvements in
Artemis in the last two weeks.  The next release will be a big step
forward in functionality and stability.  Yay!


 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/

More information about the Ifeffit mailing list