[Ifeffit] Negative Sigma^2

[pierre.lecante at cemes.fr]

one big issue is if your frequency terms are right when the amplitude  
ones are wrong, i.e. if you have computed and experimental chi  
functions reasonnably in phase while keep a not too high deltaE0 value;

if so, as Scott said it sounds like a correlation between amplitude  
terms, which is fairly common; maybe an incorrect amplitude reduction  
factor, but more likely something wrong in the model: neighbours too  
light, or with a wrong multiplicity;

if not, even worse and hard to say;

my two cents,
Pierre

ifeffit-request at millenia.cars.aps.anl.gov a écrit :

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>    1. Negative Sigma^2 (Palomino, Robert)
>    2. Re: Negative Sigma^2 (Scott Calvin)
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> Message: 1
> Date: Tue, 13 Sep 2011 13:32:46 -0400
> From: "Palomino, Robert" <rpalomino at bnl.gov>
> To: <ifeffit at millenia.cars.aps.anl.gov>
> Subject: [Ifeffit] Negative Sigma^2
> Message-ID:
> 	<B32CD18BE9D42745B75847F981879A3602D1BF05 at exchangemb3.bnl.gov>
> Content-Type: text/plain; charset="us-ascii"
>
> I am trying to fit data I recently collected and occasionally I am
> getting negative sigma squared values. Could anyone tell me what this is
> indicative of: am I using the wrong model or are my initial guesses of
> some parameters way off?
>
>
>
> Robert
>
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> Message: 2
> Date: Tue, 13 Sep 2011 14:49:30 -0400
> From: Scott Calvin <scalvin at sarahlawrence.edu>
> To: XAFS Analysis using Ifeffit <ifeffit at millenia.cars.aps.anl.gov>
> Subject: Re: [Ifeffit] Negative Sigma^2
> Message-ID: <F55C7A26-33DA-45B7-AB33-327AD4C71DE7 at slc.edu>
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>
> It's probably not initial guesses, Robert.
>
> Look at the uncertainties, and look at correlations. If you are  
> getting, say, sigma2 = -0.001 +/- 0.004 A^2, then the values aren't  
> even wrong, exactly, as the error bars are consistent with a  
> reasonable value. But sigma2 is not well-determined.
>
> If, on the other hand, the error bars are such that the value is  
> unambiguously negative, it's handy to see what correlates strongly  
> with the sigma2 in question, as that can give you some clues.
>
> Sometimes, a negative sigma2 is indicative of an incomplete model,  
> rather than one that is flat out wrong. Perhaps, for instance, there  
> are paths within the fitting range that contribute to the signal,  
> but you have not included them in your model.
>
> Of course, it could also be that the model is simply wrong. Without  
> more details from you, it is impossible to say what the problem is  
> in your particular case.
>
> --Scott Calvin
> Sarah Lawrence College
>
> On Sep 13, 2011, at 1:32 PM, Palomino, Robert wrote:
>
> I am trying to fit data I recently collected and occasionally I am  
> getting negative sigma squared values. Could anyone tell me what  
> this is indicative of: am I using the wrong model or are my initial  
> guesses of some parameters way off?
>
> Robert
> <ATT00001..txt>
>
>
>
>
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