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[Damien Caliste <damien.caliste@cea.fr>]

Hello,

Le 27/02/2009, ren <opticalcase@gmail.com> a écrit :
> what you are saying, is that for my calculation of a 2x2x2 supercell,
> i will get the same result as a 1x1x1 unit cell
Yes, but take care of the k points. The total energy of a 1x1x1
supercell is indeed one eightth of the energy of a 2x2x2 supercell if
the k point mesh in the 1x1x1 supercell is twice denser in the three
directions.

This is due to the fact that in a 2x2x2 box, you allow your system to
have plane waves with a maximum period twice the one they can have in a
1x1x1 supercell. So in the 1x1x1 you have to add k points to allow the
computation of the long period plane waves and thus have the same basis
set to compare total energy.

Damien.