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

[ankit jain]

Hello Sanjeev,

The pseudopotentials I am using are
H.pz-vbc.UPF<http://www.quantum-espresso.org/wp-content/uploads/upf_files/H.pz-vbc.UPF>
 and C.pz-vbc.UPF<http://www.quantum-espresso.org/wp-content/uploads/upf_files/C.pz-vbc.UPF>


Pseudopotential type: NORMCONS
Method: Von Barth-Car (direct fit)
Functional type: Perdew-Zunger (LDA) exch-corr
scalar relativistic


I have tried kgrid of 1*1*6 and 1*1*12, but both give negative frequencies.



On Mon, Apr 14, 2014 at 12:18 AM, Sanjeev Gupta
<physics.skgupta at gmail.com>wrote:

> what is kind of pseudopotential?
> How much k-point?
>
>
> wishes
> sanjeev
>
>
> On Sun, Apr 13, 2014 at 9:25 AM, 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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>
>
>
> --
> 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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