[Pw_forum] ibrav=2 and space_group=202: input coordinate misunderstood

[Tone Kokalj]

On Fri, 2017-09-15 at 20:22 +0000, Musil Félix wrote:
> Dear Paolo,
> 
> I am not sure to understand. The documentation on ibrav=2 shows the
> following cell convention:
> 2          cubic F (fcc)
>       v1 = (a/2)(-1,0,1),  v2 = (a/2)(0,1,1), v3 = (a/2)(-1,1,0)
> 
> which corresponds to a fcc primitive cell and the position input that
> I provide corresponds to this cell. 
> How could the position input be in simple cubic while the cell input
> is a primitive cell ?

I haven't really follow this thread, but from the above I guess the
problem arises from two possible sets of crystal coordinates for fcc,
i.e. one pertaining to cubic conventional cell and the other to
primitive fcc cell. That is, the conventional cell vectors are:

V1=(a,0,0)
V2=(0,b,0)
V3=(0,0,c)

whereas the primitive fcc cell vectors are the one that you stated
above. The "problem" now arises, because the specs of Wyckoff positions
in the input are expressed with respect to conventional V1,V2,V3
vectors, whereas pw.x prints the crystal coordinates with respect to
primitive v1,v2,v3 vectors in the output.

Best regards, 
Tone Kokalj
-- 
J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia 
(tel: +386-1-477-3523 // fax: +386-1-251-9385)