[Pw_forum] Gamma phonons in Si lattice
Stefano de Gironcoli
degironc at sissa.it
Sun Nov 18 17:18:17 CET 2007
The SC contains 8 atoms since it is four time larger than the 2-atom fcc
cell.
Therefore you MUST have 24 frequency in each point in the supercell BZ...
What they are ? where they come from ?
In order to answer this question take into consideration the fact that
the supercell BZ is four time smaller that the original one because some
q-vectors that would be inside the BZ of the 2-atom cell have become
G-vectors (i.e. equivalent to Gamma point!) in the supercell.
Check carefully your k-point sampling and other convergence parameters
in your calculation...
there should not be any imaginary frequencies in Si at equilibrium
lattice constant.
Don't forget to indicate your affiliation.
Stefano de Gironcoli - SISSA and DEMOCRITOS
Ezad Shojaee wrote:
> Hi
> if we define the Si lattice as an FCC with 2-atoms per basis, we have
> the Gamma phonons as obtained in the example#2. but it seems that if
> we define the lattice as a SC with 8-atoms per basis, we have a lot of
> frequencies which seems to have nothing in common with the first one.
> if there is no problem, what physics is behind it? did i miss
> something? and also the code seems to have problem with converging
> some of the modes in the 8-atoms per basis one(there are some negative
> freq's)
> Thanx for any comment
>
>
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