[Ifeffit] Amplitude and sigma square

[Carlo Segre]

Hi Abdul:

I agree with Pierre.  Your model is fundamentally incorrect.  It sounds 
like you are trying to fit an octahedral environment.  If there are 
multiple distances in this octahedron, it is possible that letting all of 
these distances vary freely will send you off in an incorrect minimum.

When I work with these kinds of systems, I always start with a single path 
and set N=6 in that path.  I can bring the other paths in later.  With 
this, I limit the fitting range to just the first shell (usually 1A to 
about 2A).  There should be 4 parameters available and you can let the all 
vary.

If the sigma squared goes negative and the amplitude becomes too small, 
then you need to constrain the model a bit more.  I usually fix sigma 
squared to 0.003 and then see where the amplitude reduction factor goes.

Once I am happy with the amplitude reduction factor, and I am sure that 
the deltaR is not too big, then I can start amp and deltaR close to their 
fitted values and then let sigma squared vary.  This should get you to a 
reasonable fit.

If amp still coes small and sigma squared is stable and bigger than 0.001 
then you might have a problem with self-absorption in your data (if it is 
fluorescence).

If amp is good and sigma squared turns out to be relatively large (say 
bigger that 0.003) then there is a good chance that you have good enough 
data to resolve more than one path in that first shell.  Under these 
conditions, there are several things you can do but it is complicated. 
Post again if this turns out to be the case.

Finally, the answer to your second question is that you do NOT need to fit 
longer paths if all you want to know is the first shell information.

Carlo

On Mon, 28 May 2018, Abdul Ahad wrote:

> 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
>

-- 
Carlo U. Segre -- Duchossois Leadership Professor of Physics
Interim Chair, Department of Chemistry
Director, Center for Synchrotron Radiation Research and Instrumentation
Illinois Institute of Technology
Voice: 312.567.3498            Fax: 312.567.3494
segre at iit.edu   http://phys.iit.edu/~segre   segre at debian.org