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

[ankit jain]

I have tried acoustic sum rule and it does not change negative frequencies.
I have rerun calculations with several different parameters and seems to
have modified my vc-relax output file. I am re-running it and will send it
as soon as it is done.


On Mon, Apr 14, 2014 at 1:51 AM, Prasenjit Ghosh <prasenjit.jnc at gmail.com>wrote:

> Did you try using the acoustic sum rule and see whether the imaginary
> frequencies are still there? Also can I have a look at the vc-relax output
> file?
>
> Prasenjit
>
>
> On 14 April 2014 11:10, ankit jain <ankitjain.me.iitk at gmail.com> wrote:
>
>> 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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>>>>
>>>
>>>
>>>
>>> --
>>> PRASENJIT GHOSH,
>>> IISER Pune,
>>> Dr. Homi Bhabha Road, Pashan
>>> Pune, Maharashtra 411008, India
>>>
>>> Phone: +91 (20) 2590 8203
>>> Fax: +91 (20) 2589 9790
>>>
>>> _______________________________________________
>>> Pw_forum mailing list
>>> Pw_forum at pwscf.org
>>> http://pwscf.org/mailman/listinfo/pw_forum
>>>
>>
>>
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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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> Pw_forum mailing list
> Pw_forum at pwscf.org
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