[Ifeffit] Re: A very quick question

Scott Calvin SCalvin at slc.edu
Sat Jun 19 12:24:27 CDT 2004


>Ummmm.... I think that Dave was actually refering to the peak fitting
>dialog in Athena.  I can see the use of such a thing.  A Gaussian,
>say, fitting a peak in the near edge should certainly never be -1eV
>wide or 20 eV wide.  Somehow putting boundaries on those parameters
>would be of value.

Oops! He did say Athena...it's not good for me to get my Greek gods 
mixed up. Aside from the fact that I was supposedly a Classics minor, 
some of them have thunderbolts...

>
>I concur that defending the wieghting function is the dicey part.  But
>some sorts of restraints seem like an outstanding idea to me.  Matt's
>example of bond valence sums as a restraint is one such.

Absolutely. In fact, they were a large part of my motivation for 
upgrading from feffit to ifeffit. For example, I sometimes use 
restraints when I expect a parameter to be the same as for a standard 
I have measured. If a fit to the standard yields an S02 of 0.88 +/- 
0.07, it seems reasonable to restrain the sample to have an S02 near 
0.88 using the uncertainty of 0.07 to guide my assignment of the 
weighting.

Also, fits with restraints are very useful to me as a diagnostic. If 
a fit is insisting on an S02 of 2.63, for example, I'll try 
restraining the S02 to 0.90  (weighted in such a way that +/- 0.20 
is not too heavily penalized). If the fit then happily chooses 0.87 
or something like that, I know I'm dealing with a true "false 
minimum." If, on the other hand, the fit pulls the S02 as high as it 
can given the penalty (say to 1.50) then I know it's some other kind 
of problem.

--Scott Calvin
Sarah Lawrence College





More information about the Ifeffit mailing list