Hi Scott,
Feff uses the placement of the atoms to determine the size of the muffin tin radius and the norman radius, which define where the atom is verse where the atom is not. If there are no neighboring atom then these radii become unphysically large and then the scattering amplitudes will not be accurate.
Hi Shelly, Thanks! It turns out I misunderstood what Artemis was doing--it's smarter than I gave it credit for. :) I didn't realize that if I highlighted a FEFF calculation, changed some of the FEFF.INP parameters and reran FEFF, that it would retroactively change the paths it already had to reflect the new calculation. So when I thought I was comparing the "old" feff0001.dat to the "new" one, I was really just comparing two copies of the new one! In any case, so much for my failure to use Artemis correctly. I appreciate your explanation of why FEFF needs atoms outside of the range we're planning to use; I'm clear on it now. --Scott Calvin Sarah Lawrence College