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.


On 12/21/2016 6:49 AM, Bruce Ravel wrote:
On 12/19/2016 11:01 PM, Robert Gordon wrote:
What if one were to do this FEFF calculation using the POLARIZATION card?
1 0 0 means something different if the atom positions have been rotated
by 45 degrees relative to the crystal axes along which the polarization
is defined.

Well ... you could rotate the polarization vector by 45 degrees ...

I guess I have always considered FEFF's polarization card to be an a posteriori addition to the feff.inp file -- i.e. a decision that the user makes after using Atoms to convert crystal data to a cluster in feff.inp.

B