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

[Vincenzo Verdolino]

So in this case it's obvious that is a q mash issue. I would try as it
follows:

1)make an homogeneous mash (10 10 10)
2)did you check the combination 4 4 4? That should be already enough
3)is there a particular reason why you are not using any acoustic sum rules
corrections??

relaxations looks fine

V


On Mon, Sep 1, 2014 at 2:37 AM, ankit jain <ankitjain.me.iitk at gmail.com>
wrote:

> 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>
>>>> > _______________________________________________
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>>>> > http://pwscf.org/mailman/listinfo/pw_forum
>>>>
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>>>
>>>
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>>
>>
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