Hi Norbert, I'm comparing the answers we just gave. I have almost never used FEFF for XANES...would you agree that =EXAFS= is insensitive to the difference between Ni and Cu scatterers (except, of course, for the structural differences that can be handled by fitting parameters such as delr, ss, and e0)? --Scott Calvin Sarah Lawrence College
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.