[Ifeffit] errors = 0?

Bruce Ravel ravel at phys.washington.edu
Tue Dec 9 14:09:57 CST 2003


On Tuesday 09 December 2003 02:18 pm, k-kupiecki at northwestern.edu wrote:
> Hi,
>
> I'm hoping someone out there has seen this before and can tell me what to
> do to remedy it...
>
> I've been using artemis at home all weekend on my Win 98 computer without
> incident....

You say the nicest things. ;-)

> now I'm working at home today with the same feff and data files
> working on fitting... and while Chi sq etc. is the same the +/-
> values on all my variables are now reading as 0, even though the
> values for the variables are what I would expect them to be.

It's funny that this should come up today, since I have been puzzling
over the same thing.  I have noticed that it only happens when Ifeffit
issues a large number of "fitting ..." messages followed by my
favorite Ifeffit error message: "too many iterations.  try again with
better guesses or a simpler problem."  (Can you believe it?!?!  Ifeffit
has an opinion about the quality of my guesses! ;-)

You can see Ifeffit's messages by looking at the Ifeffit tab in the
palettes window.  That's the one just to the right of the tab
displaying the fit results.  You will probably have to scroll back a
few screen's worth of text (or make the palettes window a lot taller)
to see that message since the commands issued to plot the result of
the fit require about 30 lines.  Ifeffit's error and warning messages
are printed in blue text (unless you have been monkeying around with
the color preferences).

Apparently what is happening (and I am sure that Matt can confirm or
deny this) is that Ifeffit is chugging along trying to get to the
answer when it hits the hard-wired limit on the number of iterations.
At that point ifeffit gives up the ghost and quits.  It leaves all the
fitting variables where they are and computes chi-square and the
R-factor but not the error bars.  I would guess that the fit was
pretty close to finished at that point if, as you say, the variables
and chi-square values are all reasonable.

So, what to do?  Here are some suggestions.

1.  Recompile Ifeffit with more iterations.  (That's not a very
    practical suggestion for a Windows user, but in the interest of
    completeness...)
2.  Try saving your project, quitting Artemis, restarting Artemis, and
    fitting again.  If that makes a difference, please let me know
    because it would suggest that Artemis is making some kind of
    mistake in how it uses Ifeffit's memory.
3.  Try selecting "Grab all best fit values" from the GDS menu and
    relaunch the fit, i.e. try Ifeffit's suggestion of "better
    guesses".
4.  Try setting one or more of your variables to some reasonable
    values and relaunching the fit.  That is, try Ifeffit's suggestion
    of "a simpler problem".

Artemis should, at least, screen Ifeffit's messages looking for the
"too many iterations" message and post some suitable warning.  I'll
put that on my list of things to do.  In the meantime, keep an eye on
the content of the Ifeffit buffer -- including the stuff that may have
already scrolled off screen.

Hope that helps,
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)
 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/



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