negative phonon frequency
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christian.bohley
- Posts: 5
- Joined: Thu Feb 18, 2010 10:01 am
negative phonon frequency
Hi, I try to calculate phonon frequencies of the cubic phase of BaF2. all seems to be ok, but for the phonon wavevector .25 .25 0 the two lower bands give negative phononfrequencies of around -350 cm-1. I don't understand why. Can anybody help me? The input file is attached. Thanks in advance, christian.
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BaF2.in- (4.71 KiB) Downloaded 426 times
Re: negative phonon frequency
This is probably not acoustic sum rule breaking, for once, so either the phase is genuinely unstable (ok, that won't really be the case) or you are not converged wrt some parameter...
Try doing just the soft phonon with even higher ecut/nkpt/nband. Btw, have you looked at the phonon scf convergence, to make sure it was ok? Which pseudopotentials are you using?
Matthieu
Try doing just the soft phonon with even higher ecut/nkpt/nband. Btw, have you looked at the phonon scf convergence, to make sure it was ok? Which pseudopotentials are you using?
Matthieu
Matthieu Verstraete
University of Liege, Belgium
University of Liege, Belgium
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christian.bohley
- Posts: 5
- Joined: Thu Feb 18, 2010 10:01 am
Re: negative phonon frequency
Thank you very much. I used for Ba the LDA Teter extended norm-conserving psp and for F the Troullier-Martins. Other pseudopotentials gave single negative phonon frequencies, too. What do you mean with "phonon scf convergence"? I checked the other convergences but the results did not change significantly. It is worthy to try a much higher value for nband than 12 in this case? greetings, christian
Re: negative phonon frequency
By phonon convergence I meant the D2ETOT convergence of ETOT - check the log.
Matthieu Verstraete
University of Liege, Belgium
University of Liege, Belgium