[Pw_forum] Fwd: Phonon dispersion from using matdyn.x
ankit jain
ankitjain.me.iitk at gmail.com
Mon Sep 1 17:02:56 CEST 2014
Hello Vincenzo,
Thanks for your suggestions again.
1,2) Is using (10 10 10) or ( 4 4 4) a good idea for a 2D system? My system
is non-periodic in z direction and that is why I am using (10 10 1) qgrid.
3) I tried asr as well but I still get negative frequencies. So I switched
it off just to remove one extra debugging parameter in the input file.
Thanks,
Ankit Jain
On Mon, Sep 1, 2014 at 1:25 AM, Vincenzo Verdolino <
vincenzo.verdolino at gmail.com> wrote:
> 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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>>>>
>>>>
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