[Pw_forum] Acoustic phonon frequencies in Gamma

[Uri Argaman]

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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