[Ifeffit] questions

J. M. An jman at lbl.gov
Mon Oct 27 18:28:59 CST 2003

On Monday 27 October 2003 03:10 pm, J. M. An wrote:
>/ Dear IFEFFIT helper,
/>/ I am a novice of IFEFFIT and would like to ask some questions about 
/>/ fitting variables used in the program.
/>/ I've been trying to fit my FEFF 8 calculation result to an experimental 
/>/ data for a bulk compound CdSe.
/>/ Essentially, the fitting variables, S02 and DW factor (the correlated 
/>/ Debye model) for the first-shell,
/>/ are used for the fit. I can manage to fit quite nicely with those 
/>/ variables, but the problem is that I want to use the values
/>/ of the resulting fitting variables as input for CdSe quantum dot 
/>/ calculations.  
/>/ Normally, S02 and Debye keywords in FEFF are global paramaters, not 
/>/ restricted to a single shell, so there seems to be
/>/ no way that I can put the fitting values obtained from the fit into new 
/>/ FEFF calculations.  
/>/ I was wondering if someone has experience to use bulk fitting values as 
/>/ input in a new FEFF calculation.
I don't think I quite understand what you want to do, but here are a
couple pieces of advice:

1.  It is my strong preference never to introduce S02 or sigma^2 into
    the feff calculation when I am doing exafs analysis.  It is much
    easier and much less confusing to use S02=1.0 and all sigma^2=0 in
    the feff calculation.  That way, any values that you get for those
    parameters from a fit using ifeffit are absolute numbers and do
    not require that you remember what you did in feff in order to
    interpret the ifeffit result.

2.  I do think that redoing the feff calculation might be useful at
    some time during the fit, but only if I have reason to believe
    that the *structural* parameters used in the feff calculation were
    in error relative to the real material.  If the distance between
    atoms in the real material are quite different from what you used
    in the feff calculation, then you might worry that the phase
    shifts and scattering amplitudes are not as good as they might be.

Given all that, I am not sure what you hope to accomplish by your new
feff calculation.  

Hope that helps,


Thank you for the advice.
The idea behind my research is
(1) to extract information S02 and Sigma2 by fitting my feff result to 
experiment in crystal CdSe.
(2) then to use this information to perform feff calculations for 
quantum dots CdSe of a certain shape.

In other words, the info in (1) will be used as input to quantum dot 
calculations in (2).
The reason is that no one knows how the surface terminates in the dot.
To this end, I need to know reasonable fitting values of S02 and Sigma2 
to be used in a feff input file.

Suppose that I have obtained S02 and Sigma2 by doing (1) only for the 
first shell, leaving all other shells intact.
(This is because in crystal CdSe, our calculated FEFF chi(k) using 
correlated Debye model differs from experiment for large k.)
However, my problem is how we put these fitting values in (2), since SO2 
and Sigma2 are global to all paths in Feff.

Another complication may be that in (2) I am performing a set of 
calculations for different Se absorbing atomic sites.
At the end, add all contribution from different Se sites to total 
chi(k), which is exactly what experimentalists measure.
That is why I need to put resonable S02 and Sigma2 before I essentially 
perform quantum dot calculations.

I will appreciate any further advice of yours on this, and let me know 
if this is not sufficient for you to understand what I intend to do.

Lawrence Berkeley National Lab

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