[Pw_forum] my understanding about ASR

[Eduardo Ariel Menendez Proupin]

Dear Zhang,
The ASR "cures" the dynamical  matrix or the force constant matrix., or
maybe both. The ASR can be applied by q2r.x (calculation of force constants)
or matdyn.x (calculation and diagonalization of the dynamical matrices). I
do not know , and I wuld like to know, if one should apply it twice or only
in one of the steps.
In any case, the 0 frequency at Gamma and the slopes are consistent.
However, the you must check that frequencies that are far  from zero change
little due to the ASR. Moreover, if you study the convergence vs cutoffs
(this may be long and painful), check that the convergence trend of the
frequencies is towards the values given with the ASR.

Note that negative frequencies may indicate instabilities in the structure,
e.g., atomic positions not relaxed. This does not seems to be the case in
the ouputs that you sent. Negative frequencies  at points other than Gamma
uncover the so called dynamical instability. The dynamical instability
indicates that using a larger supercell you can find relaxations of the
atomic positions (and maybe a  phase transition) that are avoided by the
symmetry imposed by the smaller supercell. I do not know if the ASR can mask
the dynamical instabilities.

Hope this helps.

eduardo

Dear Eduardo and Stefano,

Thank you for clear and careful explaination of my phonon problem. I am now
doing some tests and getting more experiences, because I am a freshman in
phonon calculation.

It seems to me that ASR can "force" the acoustic phonon at Gamma point to
zero. A naive thought is that can we just ignore it? In this case, are there
any problem that the slope of the phonon around Gamma is incorrect, because
ASR cure the problem of k=0, but what about other points near it?


-- 
Eduardo Menendez
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