[Ifeffit] atoms and the alloys

[Norbert Weiher]

Dear Mauro,

FEFF is very sensitive towards subtle changes in the atom list as caused by
introducing a vacancy or replacing a Ni by a Cu atom as you describe. Imagine
an FCC system with 50% atom A and 50% atom B. ATOMS has no a priori knowledge
how it should distribute A and B in the different shells. Unfortunately 
each of
the possible combination will give you a slightly (or heavily) different XANES
signature.
ATOMS could use a random number generator to decide how a certain point 
in space
should be occupied. In the above example it would give each atomic position in
the AB alloy a 50% chance to be either A or B. This will result in a 
FEFF input
file corresponding to only one of many possible atomic permutations. This
particular atomic configuration might be close to reality - but it might as
well be unreasonable.

For this reason ATOMS does not include the occupancy in its calculations.

However, here's a workaround:
Use atoms to generate a feff.inp for pure A. Then use a script which randomly
replaces A with B with a probability suiting your needs (be sure to use a good
random number generator). In theory you need to calculate all possible
configurations of your cluster and do a FEFF run for each of them. I tried
something similar some time ago and found that the XANES pattern converges if
you do ~40 calculations of di<fferent clusters - which is reasonable if you
have a decent computer at hand.

Norbert