More precisely: the point symmetry of the bravais lattive can be higher than the maximum symmetry of any lattie site in the crystal. For example, the point symmetry of the bravais lattice of the diamond structure is cubic, but no lattice site in the diamond structure has cubic symmetry (actually, each atomic site has tetrahedral symmetry). In this case, cubic symmetry s recovered by associating a fractional lattice translation to certain operation of the cubic group (it is said the the space group of the structure is "non symmorphic" in this case). More generally, it may happen that you simulate a system of low symmetry (a molecule, for instance) with a supercell of higher symmetry (think of using a cubic supercell for symulating an isolated water molecule, for instance) ...
SB On Aug 10, 2009, at 4:41 PM, xirainbow wrote: > Dear Gabriele Sclauzero: > > >Just one additional note of caution: kpoints.x reduces the number > of k-points (and compute > >weights) according to the symmetry of the bravais lattice only, > while the subroutine > >kpoint_grid in PW/ used by pw.x takes into account the crystal > symmetry (which can be > >lower than the lattice symmetry if you have more than one atom per > cell or non-collinear > >magnetism) > > I am confused with your statement:"crystal symmetry can be lower > than the lattice symmetry if you have more than one atom per cell". > Could you explain it more clearly? > Or could you give me a simple example? > Thank you ?? > > ____________________________________ > Hui Wang > School of physics, Nankai University, Tianjin, China > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum --- Stefano Baroni - SISSA & DEMOCRITOS National Simulation Center - Trieste http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype) La morale est une logique de l'action comme la logique est une morale de la pens?e - Jean Piaget Please, if possible, don't send me MS Word or PowerPoint attachments Why? See: http://www.gnu.org/philosophy/no-word-attachments.html -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20090810/bf370681/attachment.htm
