[Pw_forum] negative phonon frequencies for 1-D polymer chain

Sanjeev Gupta physics.skgupta at gmail.com
Mon Apr 14 06:18:47 CEST 2014

what is kind of pseudopotential?
How much k-point?


On Sun, Apr 13, 2014 at 9:25 AM, Ankit <ankitjain.me.iitk at gmail.com> wrote:

> Hello QE developers and users,
> I am trying to calculate phonon dispersion using ph.x and matdyn.x
> routines of QE and I am stuck woth negative frequencies for a while now.
> I have a 1-D chain of polyethylene which has 6 atoms in the unit-cell.
> The polymer chain is aligned in z-direction and I have added vacuum on
> x- and y- direction. I did the structure relaxation and I am getting
> total force after relaxtion as 0.000020, which I guess is sufficiently
> small.
> After relaxation, when I am trying to calculate phonon dispersion using
> ph.x and matdyn.x I am getting netting frequencies for some of the
> phonon modes. I  searched online and on QE mailing list and tried couple
> of different things:
> 1. The negative frequencies are appearing for non-gamma point as well,
> so I guess its not a ASR issue. Anyway, I am using ASR but even after
> this I am getting negative frequencies.
> 2. I reduced my tr2_ph and conv_thr to 1.0d-14 and 1.0d-15 which I guess
> are very small as suggested on QE FAQs.
> 3. I checked phonon mode shapes for modes with negative frequencies and
> they do not look like rotational mode.
> 4. I tried changing number of k points from 6 to 12 in z direction but
> this is also not the cause.
> I am really stuck here and not sure about the cause of negative
> frequencies. With vc-relax, my structure is getting relaxed which I
> guess means my structure is stable. But then I am not sure why I am
> getting negative frequencies.
> I really appreciate any help or suggestions.
> Thanks,
> Ankit Jain
> IIT Indore,
> Indore,
> India
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With Best Regards,

Dr. Sanjeev Kumar Gupta
Fulbright Post-Doctoral Scholar
Dept. of Physics
Michigan Technological University
1400 Townsend Drive, Houghton
MI 49931, USA
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