[Pw_forum] Fwd: Phonon dispersion from using matdyn.x

[ankit jain]

Hello Vincenzo,

Thanks a lot for your reply.

 "First thing that i would try is to check if phonon calculations at a q
point where matdyn gives you negative is correct or not"
How would I check this?

Also I am using tr2_ph = 1e-18 in the ph.in and conv_thr=1.0d-12 in the
pw.in file.
My kgrid is 14*12*1 in the pw.in file.

Thanks,
Ankit Jain



On Sun, Aug 31, 2014 at 1:13 PM, Vincenzo Verdolino <
vincenzo.verdolino at gmail.com> wrote:

> Dear,
>
> I have similar issues so i am not sure to be able to help you. First thing
> that i would try is to check if phonon calculations at a q point where
> matdyn gives you negative is correct or not. If yes that means that you
> k-samplin is good enough and it is only a matter of q mash for the FFT.
> Another check should be on the scf thrashold in ph.in input file
>
>
> Let me know
>
> Sent from my iPhone
>
> > On 31/ago/2014, at 17:03, ankit jain <ankitjain.me.iitk at gmail.com>
> wrote:
> >
> >
> > Hello QE developers and users,
> >
> > Recently, I am trying to calculate phonon dispersion of a 2D material
> using pw.x, ph.x, q2r.x and matdyn.x but I am getting very strange results.
> >
> > In my calculations the phonon frequencies reported by ph.x are all real
> for all non-gamma phonon q-points. But, when I use matdyn.x to calculate
> phonon frequencies at other points (other than the one for which ph.x
> calculated frequencies), I get negative (imaginary) frequencies. And these
> negative frequencies are present for non-gamma points as can be seen in
> attached frequency file.
> >
> > I am having difficulty in understanding these results and hoping someone
> could help.
> >
> > Thank You,
> > Ankit Jain
> > PhD Candidate
> > IIT India
> >
> > <pw.in>
> > <ph.in>
> > <q2r.in>
> > <matdyn.in>
> > <frequency>
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> > Pw_forum at pwscf.org
> > http://pwscf.org/mailman/listinfo/pw_forum
>
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