Re: [Ifeffit] Negative Sigma^2

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@millenia.cars.aps.anl.gov a écrit :

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Today's Topics:

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" To: Subject: [Ifeffit] Negative Sigma^2 Message-ID: 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