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

[Andrea Dal Corso]

Did you apply the 5.0.3 patch to QE 5.0.2? Without the patch, the phonon
code gives wrong frequencies for some q points. 

HTH,

Andrea


On Mon, 2014-04-14 at 01:19 -0400, ankit jain wrote:
> 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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> >
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-- 
Andrea Dal Corso                    Tel. 0039-040-3787428
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