[Ifeffit] problem with importing data between version of artemis

Bruce Ravel bravel at bnl.gov
Mon May 13 15:19:05 CDT 2013


Georges,

Sorry for the lengthy delay.  Beamline, travel, etc -- I just haven't
had a chance to look into software issues until this week.

I agree that the fits are being evaluated slightly differently in the
old and new versions of the code.  As you say, best fit values and
statistical parameters are somewhat different.  I am not sure I agree
that they are "nowhere near similar", but the disrepancy is much
larger than a mere numerical difference.  I am not yet clear what is
going on, but I am looking into it.

If I may comment a bit more broadly on the project you sent me, I have
some concerns about your fitting model.  I would not agree that your
fit is "good" using either version of Artemis.  Or perhaps I should
say that you and I may not have the same expectation of what "good
fit" means.

The central problem in your fitting model is that you are using the
same parameter for sigma^2 for all your paths, which include O, C, and
N atoms spread over a distance range from 1.69 A to 2.03 A.
Similarly, you use the same deltaR parameter for all four paths.

I understand that you have rather limited information (although you
have usable data out to at least 13 invAng), but I don't really accept
your constraint that the O atom at 1.69 should have the same sigma^2
as the N atom at 2.04.  My intuition tells me that the short O atom
should be very tightly bound to the tungsten absorber.  Consequently,
the O should have a much smaller sigma^2 than the N.

So, I question whether your model is defensible, thus I dispute that
the one fit is "good" while the other is "bad".

That said, they are significantly different.  You have uncovered a
problem that needs to be figured out and fixed.  I'll keep you posted.

B

On Monday, April 22, 2013 04:05:46 PM Georges Siddiqi wrote:
> Hi All,
> 
> I've done a fit in the old version of Artemis (0.8.014), and wanted to
> import this fit into the new version of artemis (demeter 0.9.15) that I
> have to play around with it a bit more.
> 
> The problem is, when I import the the "old-style artemis project", and do
> the same fit, the values, and fit itself are nowhere near similar to what i
> got with the old version of Artemis.
> 
> what is the cause of this discrepancy? As far as I can tell from double
> checking all the fitting parameters, windows, variables, etc, they are all
> exactly the same.  Even if I try to just do a summation in the new version
> with my best-fit values from the old version, the fit is completely
> different.
> 
> Previously when I was importing fits between versions I had no issues
> whatsoever.
> 
> I've attached the artemis project file (W EXAFS.apj) that gives me a good
> fit in the old version, and a poor fit in the new version.
> 
> thanks,
> georges


-- 

 Bruce Ravel  ------------------------------------ bravel at bnl.gov

 National Institute of Standards and Technology
 Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
 Building 535A
 Upton NY, 11973

 Homepage:    http://xafs.org/BruceRavel
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