[Pw_forum] Acoustic phonon frequencies in Gamma

[Paolo Giannozzi]

The perturbation is the same for all phonons: 3N atomic displacements,
grouped into irreproducible representations of small dimension (makes
symmetrization feasible). In some cases it may be more convenient to use
symmetry-independent atomic displacements and to use explicitly Acoustic
Sum Rule (see option gamma_gamma and code "phcg.x"). The problem of nonzero
acoustic phonon frequencies is well known and has no simple solution (other
than imposing the ASR) I am aware of


Paolo

On Wed, May 4, 2016 at 8:04 PM, Uri Argaman <argamanu at post.bgu.ac.il> wrote:

> Dear QE users and developers
> How phonon calculations is done in the acoustic branches at the Gamma
> point?  What is the perturbation?
> I know that in ph.x this frequencies can sometimes differs from zero but
> the displacements of the atoms is just rigid. I know that the deviation
> from zero is a matter of convergence.
> Thanks
> Uri Argaman
> Ben-Gurion University
> Israel
>
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-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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