[Ifeffit] Ifeffit Digest, Vol 166, Issue 12

[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.


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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## This Atoms file was generated by Demeter 0.9.25
## Demeter written by and copyright (c) Bruce Ravel, 2006-2016

title = fe foil
space = I m m m
a     =   2.83500    b    =   2.83500    c     =   2.92000
alpha =  90.00000    beta =  90.00000    gamma =  90.00000
rmax  =  10.00000    core  = fe
 # polarization = 0.0  0.0  0.0
shift =     0.00000    0.00000    0.00000
atoms
# el.     x           y           z        tag
  Fe     0.00000     0.00000     0.00000   fe