[Ifeffit] Re: Scanning "potential surfaces"

Scott Calvin scalvin at anvil.nrl.navy.mil
Tue Jul 8 13:07:41 CDT 2003

Grant Bunker says:

>I think the easiest thing to implement in feffit however would be simply
>to do a series of minimizations with (say, 1000) random starting points in
>the parameter space. This would just entail wrapping the minimization in a
>loop, and adding a way to summarize the results.
>Minimizations starting at differentpoints may end up in the same or different
>"attractors". If those corresponded to bad fits, throw them out. If they are
>adequate fits, keep them. Quite possibly more than one attractor could fit
>the data adequately. A cluster analysis of the results should indicate whether
>different solutions are equivalent (belong to the same attractor) or not.

I think there are at least two different (albeit related) purposes 
for "scanning potential surfaces" with IFEFFIT.  One is to identify 
the candidates for good fits. This is not just a matter of improving 
the fitting routines so that there is a greater probability of 
finding the true global minimum; as most of us know quite well, the 
statistically best fit is not necessarily the one that corresponds 
most closely to physical reality. Grant's suggestion sounds like a 
really good way of identifying candidates for "good" fits, and thus 
reducing the possibility that a good solution is being overlooked.

A second purpose for looking at the potential surface is to help the 
researcher better understand the relationships between the variables 
being fit. (Better than just a single correlation number, for 
example.) This may be helpful early in the fitting process, perhaps 
when a good model has not yet been found. It may also be useful for 
presentation of results to a skeptical audience! For these purposes, 
Bruce's plan to allow scanning over two variables while fitting the 
rest  sounds very useful.

--Scott Calvin
Sarah Lawrence College

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