[Ifeffit] Calculation of the doping level

[Matt Newville]

Hi Kicaj,

Just, to follow up on Bruce's comment:  A single cluster with 1 Al
absorber and no Al scatterers does not really describe "1%" or even
"0.5%" doping.  It is "0%" doping.

By "1% Al doping", you (probably) really mean that each Zn site has a
1% chance of being occupied by Al.  I think that's what you have to
simulate.  That means generated many (several hundred I would expect)
clusters with each Zn site having 1% Al occupancy, and summing the
results of those calculations.

I don't think the mean free path or size of the subcluster for the
full multiple scattering matter here, except as it guides how large a
cluster you need to consider.    The size of the cluster needed for
convergence is important, but does not really dictate an occupancy.
What you want it a set of clusters that does simulate the proper
doping level.

--Matt