[Pw_forum] problem with symmetry recognition

[Paolo Giannozzi]

On Jul 10, 2011, at 12:10 , pascal boulet wrote:

> Following Stefano's comment I need to provide ALL the atomic  
> positions,
> which I did not do! I gave only the positions in the primitive unit  
> cell (UC).

what you call "primitive", I would call "irreducible" (like the  
irreducible
segment of the Brillouin Zone)

> But then, if I provide all the positions (I guess in the full UC)

what you call "full", I would call "primitive"

> why should QE fill the others?

which others?

> In addition, I did some calculations on Mg2Si (FCC, 12 atoms in the  
> full UC)

antifluorite structure? what you call "full" here, I would call  
"conventional"

> by providing only the 3 atoms of the primitive UC. QE found 48  
> symmetry
> elements, which is correct, and the optimized cell parameters  
> seemed OK.
> So, still, is it wrong?

no, because you provided all the needed atoms (3 in the unit cell of the
FCC lattice in the [anti]fluorite structure)

> BTW, I also noticed something that looks strange to me. The  
> coordinates
> reported by QE in the output are not those I give in the input: X  
> and Y
> are the same but not Z which is scaled by c/a.

nothing strange: atomic positions in input are in crystal coodinates:

> ATOMIC_POSITIONS crystal

atomic positions in output are in xyz coordinates, divided by a:

>    Cartesian axes
>
>      site n.     atom                  positions (alat units)

P.
---
Paolo Giannozzi, Dept of Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222