[Pw_forum] IFCs and Difference between unit cell and supercell

[Paolo Giannozzi]

On Thu, 2012-10-18 at 16:50 -0400, Jingjie Zhang wrote:

> The structure is FCC with two atoms. [,,,] the a1, a2, a3 are
> conventional cell vectors or the primitive cell vectors? 

they are primitive cell vectors for the fcc lattice. 

> When I use supercell: simple cubic with 8 atoms to build silicon
> structure, the omega is different at gamma point with the omega from
> the structure with FCC.

in order to get exactly the same number, you have to use either
a converged grid of k-points, or exactly the same k-points.

> And the thing most confuse me is that the point group is different
> in this structure: 43m. Can anyone explain why this happen please?

can you explain what you mean? what is different from what exactly?

P.
-- 
Paolo Giannozzi, IOM-Democritos and University of Udine, Italy