[Wien] need help

[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.
>