[Ifeffit] Negative Sigma^2

[Scott Calvin]

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
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