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

[ankit jain]

Hello Vincenzo,

Thanks for your reply again.

As you suggested, I did phonon calculation using ph.x at one of the
negative frequency (according to matdyn.x) point.  ph.x produce all
positive frequencies for this point. Does this mean that there is something
wring in the way I am using matdyn.x for interpolating frequencies?

Also, you are right that my system is 2D system with free surface. I have
vacuum  in the z-direction to avoid any interaction. I also used only one
k-point to sample the z-direction. I have fully relaxed the structure using
multiple steps of vc-relax and relax (repeating them couple of times).  I
am attaching my pw.out file with this mail to get the idea of forces and
stress.

Thanks,
ANkit Jain


On Sun, Aug 31, 2014 at 2:57 PM, Vincenzo Verdolino <
vincenzo.verdolino at gmail.com> wrote:

> 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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>>
>>
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>
>
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