[Pw_forum] generating k point weights

[Stefano Baroni]

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
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) /  
stefanobaroni (skype)

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