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> >>>>> > _______________________________________________ >>>>> > Pw_forum mailing list >>>>> > Pw_forum at pwscf.org >>>>> > http://pwscf.org/mailman/listinfo/pw_forum >>>>> >>>>> _______________________________________________ >>>>> Pw_forum mailing list >>>>> Pw_forum at pwscf.org >>>>> http://pwscf.org/mailman/listinfo/pw_forum >>>>> >>>> >>>> >>>> _______________________________________________ >>>> Pw_forum mailing list >>>> Pw_forum at pwscf.org >>>> http://pwscf.org/mailman/listinfo/pw_forum >>>> >>> >>> >>> _______________________________________________ >>> Pw_forum mailing list >>> Pw_forum at pwscf.org >>> http://pwscf.org/mailman/listinfo/pw_forum >>> >> >> >> _______________________________________________ >> Pw_forum mailing list >> Pw_forum at pwscf.org >> http://pwscf.org/mailman/listinfo/pw_forum >> > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://pwscf.org/mailman/listinfo/pw_forum > -------------- next part -------------- An HTML attachment was scrubbed... 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