Hi everyone,The problem Bruce addresses below bears some relation to the kind of problems I've had recently in creating magnetic AF systems. In my case I introduce a new potential in the POTENTIALS list to denote atoms with opposite spin (that is, atoms which are chemically equivalent but magentically inquivalent). I had the same problem of identifying which atoms in my atomic list (in feff.inp) were the correct ones to spin-flip. In my mind this is analogous to a situation where you want two different ions on chemically identical sites.
One way of allowing Feff to consider the presence of two different
kinds of octahedral Fe sites is to create a new potential. Initially,
the feff8 potentials list might look like this:
POTENTIALS
* ipot Z element l_scmt l_fms stoichiometry
0 26 Fe_oct -1 -1 0.01
1 26 Fe_t -1 -1 1
2 26 Fe_o -1 -1 2
3 8 O -1 -1 4
You could introduce another type of iron atom by editing this to read
POTENTIALS
* ipot Z element l_scmt l_fms stoichiometry
0 26 Fe_oct -1 -1 0.01
1 26 Fe_t -1 -1 1
2 26 Fe_o1 -1 -1 1
3 8 O -1 -1 4
4 26 Fe_o2 -1 -1 1
Then, in the atoms list, change some of the atoms of potential type 2
to type 4. That will give Feff some freedom to allow the two
octohedral irons to have different potentials.
How best to distribute the new iron type throughout the atoms list is
unclear to me.
I just wanted to add that for me, the only truly reliable method I found of identifying which atoms to change the potentials of (terrible grammar, sorry), was to construct an actual, physical, ball-and-sticks model unit cell (in my case, the magnetic unit cell), and from this work out the coordinates of every atom within that cell. Then I typed ALL atomic coordinates into ATOMS 2.50, giving the, say, spin-down atoms different labels in the fifth column - my atoms.inp has columns:
atom type x-coord y-coord z-coord label (optional)
I then gave the cell space group P1 (primitive, lowest possible symmetry).The atomic list then generated in feff.inp contained atoms of the same type which had the same potential, but different labels, allowing the atoms to be spin-flipped clearly identified. A find-and-replace can then easily change the spin-flipped atoms potentials to the new value entered into the POTENTIALS list.
I realise this is a bit of work (time consuming, but not difficult), I did this for a rock salt magnetic unit cell and it is an arduous task. The spinel unit cell in particular contains a huge number of atoms. But i think its a good, honest, reliable way of distributing new atom potentials through the atomic list.
Sorry if this has missed the point of the original email, if anyone has any comments on what I've done my ears (and eyes) are open.
Cheers,Dave Eustace