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

[Prasenjit Ghosh]

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
>>>
>>>
>>> _______________________________________________
>>> Pw_forum mailing list
>>> Pw_forum at pwscf.org
>>> http://pwscf.org/mailman/listinfo/pw_forum
>>>
>>
>>
>>
>> --
>> 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
>>
>
>
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum
>



-- 
PRASENJIT GHOSH,
IISER Pune,
Dr. Homi Bhabha Road, Pashan
Pune, Maharashtra 411008, India

Phone: +91 (20) 2590 8203
Fax: +91 (20) 2589 9790
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20140414/22e01e75/attachment.html>