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

[Vincenzo Verdolino]

Hi Ankit Jain,

Germanium
 &inputph
  tr2_ph=1.0d-18
  ldisp=.true.
  nq1=10
  nq2=10
  nq3=1
  prefix='GEPH_dfpt'
  outdir='/scratch'
  fildyn='./dfpt_10x/ge.dyn'
 /


this activate a dispersive phonon calculation. If you switch the ldisp flag
to .false. , if you unset nq1, nq2 and nq3 (you just neglect those) and if
you specify q1 q2 and q3 at the very end of the input file (after the final
slash /) you can calculate the frequencies at that q point. Ex.

Germanium
 &inputph
  tr2_ph=1.0d-18
  prefix='GEPH_dfpt'
  outdir='/scratch'
  fildyn='./dfpt_10x/ge.dyn'
 /
1.0 0.0 0.0

I suggest you to pick one q point where you found imaginary frequencies
(from your frequencies output file) and just run ph.x < ph.in on it.

However I have to tell you that I am not familiar with phonon calculation
on surfaces which I believe it's what you are doing right now. Am I
correct? i see that your cell is stretched along "z" and you k-sampling is
highly non-homogeneous. This may dramatically effects your phonon
calculation. One last thing...did you relax through a vc-relax job or is
that a pseudo-crystal experimental structure?



On Sun, Aug 31, 2014 at 7:28 PM, ankit jain <ankitjain.me.iitk at gmail.com>
wrote:

> 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>
>> > _______________________________________________
>> > Pw_forum mailing list
>> > Pw_forum at pwscf.org
>> > http://pwscf.org/mailman/listinfo/pw_forum
>>
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