[Ifeffit] Fitting procedure

Adam Webb adam.webb at desy.de
Fri Apr 17 10:26:38 CDT 2009


Hi,

Perhaps I am misunderstanding something here. It sounded like the
original problem was that SO2 was too high/low for some shells. It also
sounded like the solution was then to fit each shell separately with its
own set of parameters; SO2, dr, sigma-squared.  I don't think that is
what Matt was saying but I think that was what Abhijeet implied.
However, I may have completely misunderstood what was being done.

If I did understand correctly then I have two concerns.
1. Can you justify having a different SO2 for each shell? I don't know,
I am just asking.
2. In any case, it sounds to me like a recipe for ending up with too
many free parameters. It seems to me that fitting separate pieces is
dangerous. At the very least, at some point the pieces should be put
together and the whole thing fit together.

I can see this method as being reasonable to get starting values for
each shell. However, a good model should work for the total spectrum as
well as for individual pieces. If not, I would be be concerned about the
model.

Cheers,
Adam

Kelly, Shelly wrote:
> Okay. Sure, I agree with Matt. But I don't think that this data can be
> broken into different shells....otherwise I do not believe that the
> model would fail when the entire data range is optimized together.
> 
> There could be two reasons that the model fails. Either there are
> correlations that were artificially broken by fitting the r-ranges
> separately or there is some data that is not accounted for in the model.
> 
> 
> I guess I should only talk for myself. "I do not generally accept your
> fitting method to be the best".  I did say BEST...sometimes best is not
> an option.
> 
> Shelly
> 
>> -----Original Message-----
>> From: ifeffit-bounces at millenia.cars.aps.anl.gov [mailto:ifeffit-
>> bounces at millenia.cars.aps.anl.gov] On Behalf Of Matt Newville
>> Sent: Friday, April 17, 2009 9:23 AM
>> To: XAFS Analysis using Ifeffit
>> Subject: Re: [Ifeffit] Fitting procedure
>>
>> Shelly wrote:
>>
>>> ....and what we are trying to say is that your method is not
> generally
>>> accepted as the best way to model data.
>> We are?
>>
>> If I understand Abhijeet's approach (first fit the first shell, then
>> fix those parameters, change the R-range to the second shell and fit
>> that),  it is exactly what I would recommend.
>>
>> --Matt
>>
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