[Ifeffit] High SO2
SCalvin at slc.edu
Fri Apr 10 08:26:55 CDT 2009
Well, I do know that many people do it that way, so it's not "wrong."
But there are a number of problems with it:
* You're locking in errors, rather than using additional data to
improve your fit. After all, the fit to the first shell has
uncertainty, given by the error bars. But you're choosing the center
point of the range and locking it in. By doing that, you're then
distorting the fit to the next shell.
* You're ignoring "leakage" from the second shell when you fit the
first shell. The second-shell paths have some width (look at them in
Artemis to see this). Inevitably they have some tail that extends into
the first shell. A fit to the first shell therefore has a systematic
bias. If all you're doing is fitting one shell, then it may be small
enough to live with it. But if you're adding a second shell anyway,
why not let it inform the fit?
* It becomes difficult to keep track of degrees of freedom. By doing
fits in layers, you of course aren't actually reducing the number of
parameters you're fitting, but ifeffit doesn't know that. The reduced
chi-square statistic is no longer reduced by the right number of
degrees of freedom, so you lose the ability to use it to compare fits.
And if you decide during the fitting of the second shell to change the
k-range, degrees of freedom can get quite confusing.
Maybe someone who uses this strategy should speak up for it; I'd like
to understand what the advantages are.
Just to be clear, what I'm suggesting instead is that you fit the
first shell alone first, but when you add the second shell you
continue to allow the first shell parameters to vary.
In any case, either way that you do it, you should be using the S02
from the first shell for the subsequent shells. Otherwise you're not
taking advantage of your understanding of the physical meaning of the
Sarah Lawrence College
On Apr 10, 2009, at 5:52 AM, abhijeet gaur wrote:
> Hello Scott Sir,
> I got that ifeffit basically works on the
> principle of making the theoretical data look similar to
> experimental data. But what I know about fitting is that we have to
> first fit the first shell and then keeping the first shell parameter
> constant , we go for the next shell. Is it not right. Please tell me
> Abhijeet Gaur
> Ifeffit mailing list
> Ifeffit at millenia.cars.aps.anl.gov
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