[Pw_forum] Xcysden cann't give all atoms in the conventional cell.

[Tone Kokalj]

On Mon, 2008-06-09 at 19:36 +0800, zhaohscas wrote:
> Hi all,
> 
> I noticed the following things in Xcysden: it cann't give all atoms in the conventional cell based on the 
> PRIMVEC, CONVVEC, and PRIMCOORD parameters only.  In detail, the space group number of a specific crystal 
> is of importance in determining the atoms in a cell.  From the Xcrysen's descriptions, it just use the 
> above three parameters to generate the atoms in the conventional cell.  So, I think it is inadequate to 
> decide all the atoms in the conventional cell corresponding to a primitive cell.

First, note that the PRIMVEC and CONVVEC share the same origin (0,0,0).
If one have ALL (and not only irreducible ones) atoms within the
primitive unit cell and the associated latice vectors, then an infinite
perfect crystal can be generated. Once the crystal is build there is an
infinite number of choices for the cell. One can pick the one he/she
likes. Some will be bigger, some will be smaller. From this perspective
convetional cell is nothing but a small supercell. So the
primvec/convec/primcoord should be sufficient.

If you have find a structure that xcrysden fails to generate proper
convetional-cell build structure, I woule appriciate if you send me the
associated file, so that I can investigate.

Best regards, Tone

PS: if you would like to know how the alrgorithm works, here is the
simplified recipe: xcrysden generate "big-enough" portion of the crystal
based on primvec/primcoord, and then determines the atoms within the
convvec specified cell. The program of course performs some checks that
convvec are OK.

-- 
Tone Kokalj <tone.kokalj at ijs.si>
J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia (tel:
+386-1-477-3523 // fax:+386-1-477-3822)