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

[ankit jain]

Hello Prasenjit,

I am getting negative phonon frequencies for non-gamma points also. After
structure optimization, the total force is 0.000020 Ry/au and pressure is
-0.24 kbar.

I am using ecutwfc of 100 Ry, which is converged with respect to the total
energy of the system. I am not able to do convergence with respect to
phonon modes as some modes have negative frequencies. Is there a way to do
ecutwfc convergence for phonon modes without calculating phonon frequencies?

I am atatching my pw.in, ph.in pw.out and ph.out files (which I forgot to
attch in my original mail) here.

Thanks,
ANkit Jain


On Mon, Apr 14, 2014 at 1:29 AM, Prasenjit Ghosh <prasenjit.jnc at gmail.com>wrote:

> 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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>> Pw_forum at pwscf.org
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>>
>
>
>
> --
> PRASENJIT GHOSH,
> IISER Pune,
> Dr. Homi Bhabha Road, Pashan
> Pune, Maharashtra 411008, India
>
> Phone: +91 (20) 2590 8203
> Fax: +91 (20) 2589 9790
>
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