[Ifeffit] Structures with multiple crystallographic sites

[Bruce Ravel]

>
> Thanks, Matt! Yes, that's exactly what I tried doing. It's a simple cubic
> lattice with 20 atoms per unit cell, 8 of one type (Co1) and 12 of the
> other (Co2). So I did two FEFF runs in the same Artemis project, with Co1
> and Co2 as the core atom, and multiplied their respective amplitudes by 0.4
> and 0.6. Then I noticed that if I calculate a sum of all included paths I
> get a different FT spectrum than if the sums are calculated separately for
> the two FEFF runs and averaged (with correct weighting) outside of Artemis.

I usually do something like:

  guess amp = 0.9

then for a path in the first feff calculation:

  s02 = 0.4 * amp

and for a path in the second feff calculation:

  s02 = 0.6 * amp


> Of course, it is a bit late here and I could be missing some obvious
> mistake. On the other hand, I am trying for 1.6-5.5 angstrom fitting range,
> which involves 70+ paths for Co1 core and ~120 for Co2. Is it possible that
> Artemis does not like to deal with too many paths and/or variables? I
> better go check the manual... :o) Regards.
> Vadim.

Getting the math expressions right for 190 paths is tedious and error
prone, but there artemis and ifeffit should deal with that many paths
without a problem.  There is the caveat that you might start pushing
ifeffit's built-in memory lmitations, so you should save your project
often. ;-)

B



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