[Pw_forum] Strange phonon branches appear in phonon dispersion

Stefano de Gironcoli degironc at sissa.it
Wed Dec 12 11:25:50 CET 2007

your phonon branches are really strange...
with two atoms I would expect 6 modes per each q-point


Viet Bac wrote:
> Dear Pw_users,
> I am trying to do calculation phonon dispersion and EPC for 1D 
> Aluminum chain. I got the results which are strange phonon branches 
> appearing. In the case of 2 atom-cell, I got 4 phonon branches, two of 
> them are Optical and Acoustical phonon branches and two others, which 
> have zero and negative values, are unknown for me. In the case if I 
> have 1 atom-cell, that strange branch also appears. Could any body 
> explain for me what kind of frequencies here?
> I was surfing this forum and FAQ and I found many discussion about 
> negative frequencies, but I still cannot find the answers for me.
> Bac

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