[Ifeffit] Amplitude and sigma square (Abdul Ahad)

28 May 2018

      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@millenia.cars.aps.anl.gov a écrit :
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Today's Topics:
1. Amplitude and sigma square (Abdul Ahad)
----------------------------------------------------------------------
Message: 1
Date: Mon, 28 May 2018 14:16:39 +0530
From: Abdul Ahad 
To: ifeffit@millenia.cars.aps.anl.gov
Subject: [Ifeffit] Amplitude and sigma square
Message-ID:

Content-Type: text/plain; charset="utf-8"
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

[Ifeffit] Amplitude and sigma square (Abdul Ahad)

Pierre Lecante