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

[Prasenjit Ghosh]

For which q-point are you getting the imaginary modes? If it is at gamma,
then it may be related to the instability in the structure, if it is at
other q-points, may be lattice parameter is not properly optimized. How
large is the stress you are getting at the end of the vc-relax calculation
(when the code recalculates the total energy, stress and forces with the
optimized lattice parameters & atomic positions). Also have you checked the
convergence of the phonon modes with ecutwfc because typically the phonons
converge much slowly with the size of the basis set than other parameters?

With regards,

Prasenjit


On 13 April 2014 21:55, 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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-- 
PRASENJIT GHOSH,
IISER Pune,
Dr. Homi Bhabha Road, Pashan
Pune, Maharashtra 411008, India

Phone: +91 (20) 2590 8203
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