[Wien] need help

[Peter Blaha]

The procedure you describe is absolutely correct, but one word of warning:

I have not checked experimental papers, but if they say Co2MnAl has SG 225
it can only mean that Mn and Al are disordered, i.e. both have an occupancy
of 0.5 (at each of the "8c" positions). Something like this cannot be used
in theoretical simulations, because at a certain position only ONE particular
atom can sit.
The first and simplest way out is what was described below, namely removing the
"spacegroup" and selecting "F" centered lattice, and additionally "split" the positions
putting a Cr at one and Al on the other side. Consequently, this would lead to
SG 216.  But remember: if experiment says it is SG 225, this cell is probably not a good
approximation to it as it corresponds to a particular ordering, which is
apparently not present.

The proper way would be to create a bigger "supercell" and distribute the atoms
"randomly" (and to investigate the effect of order/disorder).

Now to your particular question:
 > I have a general question here that goes beyond this topic.  If sgroup reduces the cell, should we stay with the spacegroup (F in this case) or use the reduced cell?  I
 > think one should stay with spacegroup F, because the calculation from the reduced cell would be different.  If haven't got around to doing some calculations to test this.
 > If the reduced cell gives the same results, this would likely be preferred as the simulation would likely be faster using symmetry.  Any comment on this would be 
appreciated.

If sgroup REDUCES the cell, you MUST follow this recommendation. Otherwise you will get problems
in symmetry (or at latest in dstart (cannot find rotation matrix ...)).

Take a FCC Cu lattice, create a "P" 1x1x1 supercell. This generates a 4 atom cell (0,0,0),(.5,.5,0)
and two more permutations.
nn will find that all 4 atoms have identical distances and thus make them equivalent (MULT=4)
sgroup will find that the actual lattice is FCC and SG 225, reducing the cell by 4 with just
one atom at (0,0,0) (back to the original FCC cell).  If you neglect sgroup,
symmetry will probably complain that MULT not correct, because the symmetry operations and multiplicities
are not compatible with each other.

There could be other cases, where sgroup does NOT REDUCE the cell, but CHANGES the lattice; eg. from a
FCC lattice to R or B,... In such cases, you may neglect sgroup and stay in the original lattice.
However, crosscheck that sgroup did not change the number of non-equivalent sites nor their multiplicities.
(It has the same total number of atoms in the cell, only the lattice (and maybe thus also a,c,b,angles)
has changed to go to a "standard crystallographic" setting.
In addition, symmetry must give the same NUMBER of symmetry operations as sgroup.



Am 14.01.2013 20:49, schrieb Gavin Abo:
> Someone, please correct me if I'm wrong.
>
> For example, let full heusler Co2MnAl with 225 spacegroup be:
>
> a = b = c = 5.694 angstrom
> alpha = beta = gamma = 90 deg
> Co (0,0,0) and (0.5,0.5,0.5)
> Mn (0.25,0.25,0.25)
> Al (0.75,0.75,0.75)
>
> In StructGen, spacegroup 225 is selected.  Then, the following is used:
>
> a = b = c = 5.694 angstrom
> alpha = beta = gamma = 90 deg
> Co 1 (0,0,0)
> Co 2 (0.5,0.5,0.5)
> Mn (0.25,0.25,0.25)
>
> The struct file is saved.
>
> StructGen uses inequivalent positions.  The positions Mn (0.25,0.25,0.25) and Al (0.75,0.75,0.75) are equivalent in spacegroup 225, so we cannot place Al (0.75,0.75,0.75)
> in spacegroup 225. StructGen is then used to edit the saved struct file.  Spacegroup F is selected, Save Structure, continue editing, and position (0.75,0.75,0.75) is
> "split".  The Z value is deleted and renamed from Mn to Al for position (0.75,0.75,0.75).  The structure file is saved.  In xcrysden, the spacegroup 225 structure with Mn
> in all 8c (0.25,0.25,0.25) sites looks the same as the spacegroup F structure except half the 8c sites are now occupied by Mn at (0.25,0.25,0.25) and the other by half by
> Al atoms at (0.75,0.75,0.75).
>
> I have a general question here that goes beyond this topic.  If sgroup reduces the cell, should we stay with the spacegroup (F in this case) or use the reduced cell?  I
> think one should stay with spacegroup F, because the calculation from the reduced cell would be different.  If haven't got around to doing some calculations to test this.
> If the reduced cell gives the same results, this would likely be preferred as the simulation would likely be faster using symmetry.  Any comment on this would be appreciated.
>
> This particular case doesn't seem to be a good example, because the cell is not reduced by sgroup finding spacegroup 216 that looks equivalent to the spacegroup F
> structure.  I'm still learning, so hope that makes sense.
>
> On 1/14/2013 6:05 AM, idris.09 idris wrote:
>> Dear user of wien2k i need help as i am not able to define a structure file for full heusler alloy X2YZ (space group 225) type structure. where x is at (000) and
>> (1/2,1/2,1/2, ). Y atoms at (1/4,1/4,1/4) and Z atoms occupy (3/4,3/4,3/4) positions. when i define these positions and click to view structure it displays wrong
>> structure. however it define the same structure for space group 216. but i need the structure file with space group 225.
>>
>
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-- 
-----------------------------------------
Peter Blaha
Inst. Materials Chemistry, TU Vienna
Getreidemarkt 9, A-1060 Vienna, Austria
Tel: +43-1-5880115671
Fax: +43-1-5880115698
email: pblaha at theochem.tuwien.ac.at
-----------------------------------------