Robert, You have me confused. You seem to be saying that using the correct space group symbol is important. I couldn't possibly disagree. Raj's original question had to do with the fact that two different versions of atoms, which used different algorithms for interpreting the crystal data, resulted in clusters that were rotated relative to one another. My answer, appropriate (I think) in that case, was to shrug. You seem to be commenting on something unrelated to the original question, but I'm not understanding the point. B On 12/21/2016 01:13 PM, Robert Gordon 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.
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
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-- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Science Group at NSLS-II Building 743, Room 114 Upton NY, 11973 Homepage: http://bruceravel.github.io/home/ Software: https://github.com/bruceravel Demeter: http://bruceravel.github.io/demeter/