[Ifeffit] Ifeffit Digest, Vol 166, Issue 12

[Matt Newville]

Robert,


On Wed, Dec 21, 2016 at 12:13 PM, Robert Gordon <ragordon at 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
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