[Wien] Laves C15 symmetry reduction

[Peter Blaha]

The F cell is the correct cell !!!

A "P" cell of an actually FCC cell is automatically a "supercell".

However, with wien2k you cannot run (super)cells with the same symmetry 
as the original cell, as wien2k has implemented on 48 symmetry 
operations. Suppose you create a 10x10x10 cell and want to keep 
symmetry: it means that now 1000 atoms would be "equivalent" and there 
must be 1000 symm.ops which transforms the equivalent atoms into each 
other, ....

So if you want to use a cell which has by construction 4 times more 
atoms, you have to break the symmetry. Depending on how you do that, you 
can end up in various subsymmetries, either a R cell (the R cell is the 
primitive cell of every F-centered cell anyway), or, eventually in P1.
If you are clever, you can probably keep the simple cubic lattice and 
some of the symmetry operations, but you have to work this out and 
split/group equivalent atoms yourself.



On 02/27/2017 03:08 PM, Guo-ping Zhang wrote:
> Dear Wien2k users and developers,
>
> I encounter a problem with Wien symmetry operation that I could not find
> an answer from mailing list.
>
> I have an intermetallic rare-earth compound, CeFe2, which has C15 cubic
> Laves phase of MgCu2, with group symmetry No. 227, and with Z=8 in the
> conventional cell. But I need to contruct a simple cubic cell (P)
> instead of fcc (F) one given by Wien2k. So I use x supercell to generate
> P structure and then rename one Ce by Ce1 and one Fe by Fe1 or any
> combination of them or increasing lattice constant along one direction.
> If I allow Wien to use its generated structure, I ended up with
> Rhombohedral structure, or even worse without symmetry. I have no
> trouble with simple fcc structure, but have no experience with Laves phase.
>
> Here is my structure
>
> F                            2 27_F
>              RELA
>  13.800670 13.800670 13.800670 90.000000 90.000000 90.000000
> ATOM  -1: X=0.12500000 Y=0.12500000 Z=0.12500000
>           MULT= 2          ISPLIT=-2
>       -1: X=0.87500000 Y=0.87500000 Z=0.87500000
> Ce         NPT=  781  R0=.000010000 RMT=   2.50000   Z:  58.00000
> LOCAL ROT MATRIX:    1.0000000 0.0000000 0.0000000
>                      0.0000000 1.0000000 0.0000000
>                      0.0000000 0.0000000 1.0000000
> ATOM  -2: X=0.50000000 Y=0.50000000 Z=0.50000000
>           MULT= 4          ISPLIT= 8
>       -2: X=0.50000000 Y=0.75000000 Z=0.75000000
>       -2: X=0.75000000 Y=0.75000000 Z=0.50000000
>       -2: X=0.75000000 Y=0.50000000 Z=0.75000000
> Fe         NPT=  781  R0=.000050000 RMT=   2.42000   Z:  26.00000
> LOCAL ROT MATRIX:    0.0000000-0.7071068-0.7071068
>                      0.0000000-0.7071068 0.7071068
>                     -1.0000000 0.0000000 0.0000000
>
>
> I would appreciate it if anyone can give me some hints on this as what
> is the correct procedure to generate a correct structure.
>
> Thank you very much!
>
> Best regards,
>
> Guoping
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-- 

                                       P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-165300             FAX: +43-1-58801-165982
Email: blaha at theochem.tuwien.ac.at    WIEN2k: http://www.wien2k.at
WWW:   http://www.imc.tuwien.ac.at/TC_Blaha
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