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 <[email protected]> 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 > > _______________________________________________ > Pw_forum mailing list > [email protected] > http://pwscf.org/mailman/listinfo/pw_forum > -- 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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