ABINIT Discussion Forums

On Fri, Apr 9, 2010 at 7:42 AM, kondaiah samudrala <konda.physics@gmail.com> wrote:

Dear all,

I am trying to calculate phonon band structures for CbN.I follow the same way as indicated in the tutorial RF2.But i got negative phonon frequencies.here i attached the my all input files,please help me to solve this problem

Thank you to all

with regards
S.Appalakondaiah

Negative phonons modes mean:
1) the phase is really unstable (you have experimental data?) It could be that at 0K the room temperature phase is unstable (by default you calculate the phonons for low T and no thermal effects in abinit)
2) your pseudopotentials are not good enough. Try another, and make sure you are converged wrt ecut (the hgh are very hard pseudos in general) and nkpt
3) even if the potentials are ok, they may be generating a breaking of the acoustic sum rule (see other posts in this section of the forum)
4) in your case, check the magnetism, something may be happening with the Co


Matthieu


"By now, it's out of the frying pan, into the ashes."

Hi,Matthieu!
in your post,you said that " even if the potentials are ok, they may be generating a breaking of the acoustic sum rule ".
But how i could know a potential breaking the acoustic sum rule?
If i generate a pseudopotential by OPIUM, Can i know whether the potential breaking the acoustic sum rule or not by input file of OPIUM?

thinks ,
hhwj340

Hi

You will know that your pseudopotential breaks the acoustic sum rule if the first three frequencies in your output file (the acoustic modes, hence the name) are negative.

You can use anadbb to restore the asr so it shouldn't be a problem.

Boris