[Pw_forum] symmetry operations

[Miguel A. Salvadó]

Yes, I am using CRYSTAL coordinates. 3/4 1/4 3/4 and 1/4 1/4 1/4 differs in
a 1/2 0 1/2 translation and then they are equivalent in a F-centered cell.
In fact 0 0 0 and 3/4 1/4 3/4 are the coordinates for the position 8a in
International Tables of Crystallography!!

Best wishes,
Miguel

-----Mensaje original-----
De: pw_forum-admin at pwscf.org [mailto:pw_forum-admin at pwscf.org] En nombre de
degironc
Enviado el: martes, 23 de mayo de 2006 19:44
Para: pw_forum at pwscf.org
Asunto: Re: [Pw_forum] symmetry operations

Are you using CRYSTAL coordinates?
in this case 3/4 1/4 3/4 and 1/4 1/4 1/4 are NOT equivalent.
the former is wrong and corrseponds to the following cartesian coordinates
(from the PWscf output)
         2           Si  tau(  2) = (  -0.7500000   0.5000000   0.5000000  )
which are obviously wrong.
stefano


Miguel A. Salvadó wrote:

>  
>
> Dear PWusers:
>
>  
>
> I do not understand how PWSCF generate the symmetry operations from 
> ibrav and crystal coordinates.
>
> A simple example: C diamond (Fd-3m) with C at 8a (0,0,0) (Cell choice 1).
>
> If I run a calculation with ibrav 2 (cubic F-centered) and two C atoms 
> at 0,0,0 and 3/4,1/4,3/4 the program found 16 symmetry operations.
>
> But if I change the second position to the equivalent one 1/4 1/4 1/4, 
> then the program found successfully 48 symmetry operations.
>
> I have found this problem with more complex structures and it is not 
> easy to check all the possibilities of equivalent positions combinations.
>
> Any kind of help will be welcome,
>
> Best wishes,
>
>  
>
> Miguel A. Salvadó
>
>  
>
>  
>
> Miguel A. Salvadó
>
> Dpto. Quimica Fisica y Analitica
>
> Universidad de Oviedo
>
> Spain
>
> E-mail: mass at uniovi.es
>
>  
>
>  
>
>  
>

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