[Pw_forum] Gamma phonons in Si lattice

[Stefano de Gironcoli]

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.

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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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