Robert,


On Wed, Dec 21, 2016 at 12:13 PM, Robert Gordon <ragordon@alumni.sfu.ca> wrote:

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