Robert,
On Wed, Dec 21, 2016 at 12:13 PM, Robert Gordon
Hi Bruce,
The atoms.inp file attached is for bct Fe. In symmetry I4/mmm, the resulting feff input has the coordinates rotated, while in Immm the resulting coordinates are not rotated relative to the crystal axes (i.e. in Immm the 2nd near neighbour is obvious as a lattice constant away in each direction).
So, with no polarization, the results for scattering paths are the same (using CRITERIA default), as expected Path # Degen Reff Scattering Path Type Rank I4/mmm Rank Immm 1 8 2.48 fe1 ss 100 100 2 4 2.835 fe2 ss 37.56 37.56 3 2 2.92 fe3 ss 17.59 17.59 4 16 3.897 fe1-fe1 ms 3.42 3.42 5 32 3.897 fe1-fe2 ms 12.83 12.83 7 16 3.94 fe1-fe3 ms 6.37 6.37
Now consider with POLARIZATION 1 0 0 (and 1 1 0 for I4/mmm)
# Degen Reff Scatt. Path Type I4 1 0 0 I4 1 1 0 Im 1 0 0
4 2.48 fe1 ss 100
4 2.48 fe1 ss 7.89
8 2.48 fe1 ss
100 100
4 2.835 fe2 ss 55.45
2 2.835 fe2 ss
62.05 62.05
16 3.897 fe1-fe1 ms 4.41 9.90 15.43
16 3.897 fe1- fe2 ms 19.03 21.58 21.59
For that first multiple scattering path, the scattering angles are 36.1/90 for I4(1 0 0), 55.1/55.1 for I4(110) and for Im(100) they are 55.1/124.9. The ranking of this path is not the same, and the plots for this path differ as well - in shape as well as amplitude.
Conceptually, the Immm model calculation has the atoms aligned with the crystal lattice and the I4/mmm has the atoms rotated. FEFF sees a difference, and I would take the Immm result as the correct one based on the positions of the atoms in the cluster relative to the axes.
-R.
Doesn't that assume that the coordinate system for the feff.inp file (and, so the POLARIZATION card) is such that X, Y, Z listed in feff.inp are along the A, B, and C crystallographic directions of the original crystallographic description? I think the original post here was noticing that this was not always the case, and Bruce confirmed that this was not always the case. Was it ever guaranteed to be the case that XYZ in the feff.inp file was along the original crystallographic axes? I honestly don't know. It does seem like a decent convention to use, and I'd be surprised to see the coordinates rotated by an arbitrary rotation matrix, but I can believe that some structures could end up being rotated by 45 or 60 degrees from the ones specified in the CIF or Atoms file. The POLARIZATION card goes in the feff.inp file, not in the atoms.inp file, and the original crystallographic description of the structure is pretty much lost at that point -- feff.inp contains a cluster of atom, not a crystal structure. The POLARIZATION card has to be relative to the cluster of atoms listed. --Matt Newville