[Ifeffit] Re: Scanning "potential surfaces"
weiher at tech.chem.ethz.ch
Wed Jul 9 02:41:25 CDT 2003
thanks first for the many ideas you suggested. I must admit that I like all of
them. When looking for a global minimum, the variant with some random
starting points is for sure a good way of doing the job. With this one can
also get a first impression, how many "attractors" can be found in the given
The "systematic" scan on the other hand is really a good way of better
understanding the correlation between the different parameters.
So I will try to play around with both ideas.
Bruce: thanks for the tip with the ifeffit script output - I'll use it for
Have a nice day,
On Tuesday 08 July 2003 20:07, Scott Calvin wrote:
> 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
> Ifeffit mailing list
> Ifeffit at millenia.cars.aps.anl.gov
Dr. rer. nat. Norbert Weiher (norbertweiher at yahoo.de)
Laboratory for Technical Chemistry - ETH Hönggerberg
HCI E 117 - 8093 Zürich - Phone: +41 1 63 3 48 32
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