[Pw_forum] symmetry operations

[degironc]

quoting from "Wyckoff" about Diamond
BEGIN{QUOTE}
II,i1:
The unit cubes of crystals with the {\it diamond structure} have eight atoms
in the position of $O_h ^7$ ($Fd3m$):
 (8a) 000;0 1/2 1/2; 1/2 0 1/2; 1/2 1/2 0;
         1/4 1/4 1/4; 1/4 3/4 3/4; 3/4 1/4 3/4; 3/4 3/4 1/4
or more briefly
  (8a) 000; 1/4 1/4 1/4; F.C.
END{QUOTE}

This is fine ! BUT this refers to the cubic  (SIMPLE CUBIC) structure 
with 8 atoms per cell (the "F.C."  label tells you to replicate the 
previous 2-atom coordinates by  shifting them by
0 1/2 1/2; 1/2 0 1/2; 1/2 1/2 0.

If you chose the FACE CENTERED CUBIC cell you have only two atoms per 
unit cell
and different fundamental lattice vectors, hence 1/4 1/4 1/4 and 3/4 1/4 
3/4 in crystal coordinates (w.r.t  to these FCC fundamental lattice 
vectors , NOT to the simple cubic ones) are not equivalent.
If you chose CARTESIAN coordinates in unit of alat(the default for 
PWscf) then the two positions are obviously equivalent due to 
translational symmetry.

best regards,

stefano


Miguel A. Salvadó wrote:

>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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