[Ifeffit] Amplitude and sigma square (Abdul Ahad)

[Pierre Lecante]

Hi Abdul,

In my opinion, the short answer is that your model is wrong; amplitude, 
coordination number and sigma square are strongly correlated;

if your model is not adequate or if values are too far from the 
convergence zone, non physical effects appear;

usually, very small or negative values for sigma2 mean coordination 
number too low, but very small amplitude means the reverse!

I suggest to start with a plain simulation (all parameters locked to 
expected values) and check that there is not something weird in 
different k zones, which might eventually indicate unexpected species 
close to the central atom;

my 2 cents,

Pierre


Le 28/05/2018 à 19:00, ifeffit-request at millenia.cars.aps.anl.gov a écrit :
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> Message: 1
> Date: Mon, 28 May 2018 14:16:39 +0530
> From: Abdul Ahad <matrixabdulahad at gmail.com>
> To: ifeffit at millenia.cars.aps.anl.gov
> Subject: [Ifeffit] Amplitude and sigma square
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> 	<CAEFSGt6r3xbjc2zPk1fc2bGmKwgb_AVVcfrWa198kmY8QkpMJA at mail.gmail.com>
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>
> Hi all,
>
> I have couple of questions.
> 1. I found in many tutorials that the value of amp should lie between 0.7
> and 1. But when I refined it goes to 0.15, what's wrong with it.
>
> 2. The value of sigma2. Some time at high k, value, it turns to negative
> but small. What I suppose to do?
>
> 3. If I need to know the behavior of octahedra distortion with temperature,
> and it is lies within the first shell. So what is the need of higher shell
> fitting.???
>
> Thanks in advance
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