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

[ankit jain]

I will try changing the pseudopotential and repeat the calculations. I will
report back to you as sson as my calculations are finish (will probably
take a day to finish structure relaxation).


On Mon, Apr 14, 2014 at 2:12 AM, Sanjeev Gupta <physics.skgupta at gmail.com>wrote:

> good.
> can you take one more test to  change the pseudo and see. May be I am
> wrong but see.
>
>
>
>
>
>
> On Sun, Apr 13, 2014 at 11:02 PM, ankit jain <ankitjain.me.iitk at gmail.com>wrote:
>
>> 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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>>
>>
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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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