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

[Sanjeev Gupta]

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
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> 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
>>>>
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>>>>
>>>
>>>
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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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