newville at cars.uchicago.edu
Mon Oct 27 20:01:26 CST 2003
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.
More information about the Ifeffit