[Ifeffit] questions

[Matt Newville]

Dear Yoon, 

On Mon, 27 Oct 2003, J. M. An wrote:

> 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.
 
If you are trying to forward-model EXAFS for particular
configurations, than applying a best-fit S02 to the full calculation
could make sense.  Similarly, applying a best-fit Debye Temperature
could be useful.  It may not be perfectly applicable to quantum
dots, but if it's all you've got, it's better than nothing.

But if I might suggest another approach, I would replace Feff's
primitive sum-of-paths with the sum-over-paths in Ifeffit and/or
Feffit.  In Ifeffit, you would simply define a set of paths, and use
the ff2chi() command, In Feffit you can simply not specify any data
to fit.  With this approach, you can do a series of Feff
calculations with the standard S02=1, sigma2=0 (these parameters
aren't applied to the feffNNNN.dat files anyway), and still easily
average over paths from different configurations, and can apply
disorder and amplitude reduction factors as you like.

--Matt