Hi Carlo, Hmmm...my initial reaction was that, rather than loading Artemis with a new option, it's better just to "fake it" by using a low-symmetry space group with a large lattice parameter and then truncating the cluster size at smaller than the unit cell. But I just tried this, and discovered an interesting problem, which you may have come across as well. What I did was attempted to create an octahedral cluster by using a space group of P m 3 m (simple cubic) with a=10, an Fe at 0 0 0 absorbing, and an oxygen at 0.2 0 0 (thus an Fe-O distance of 2 angstroms). The cluster size is set at 5. As expected, that generates an octahedral cluster of oxygens around the Fe as a FEFF.INP file. But here's the problem: Artemis includes a potential for scattering atoms of Fe, and there aren't any in the list. FEFF, in turn, doesn't like that and won't run, causing Artemis to give the error message: " Feff 6L.02 No atoms or overlap cards for unique pot 1 Cannot calculate potentials, etc. Fatal Error: at RDINP This error is probably due the Atoms list in the the Feff input file being too short to contain an example of each unique potential. Try increasing the Rmax on the Atoms page and re-running both Atoms and Feff." This can be fixed by editing the feff.inp file to remove the Fe potential 1, but that's not enough--the oxygen scatterers then have to have their potential renumbered to 1 to make things work. That's a lot of work to do what should be simple. So I vote for changing the way Artemis works in such circumstances so that less editing is needed: if the absorbing atom is not present in the scattering list, then it shouldn't have a second entry in the potential list. As far as the other special arrangements (tetrahedral etc.), if you're not interested in going beyond nearest neighbor direct scattering EXAFS analysis, then there is no difference between them except for a degeneracy factor which can easily be inserted later in the process (i.e. only one FEFF path is needed). --Scott Calvin Sarah Lawrence College
Bruce:
Occasionally, I don't know or don't care what the extended crystallographic environment of the absorber. All I want to do is