[Ifeffit] Ifeffit Digest, Vol 166, Issue 12

Bruce Ravel bravel at bnl.gov
Wed Dec 21 12:51:06 CST 2016


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 at 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/


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