I have tried acoustic sum rule and it does not change negative frequencies. I have rerun calculations with several different parameters and seems to have modified my vc-relax output file. I am re-running it and will send it as soon as it is done.
On Mon, Apr 14, 2014 at 1:51 AM, Prasenjit Ghosh <prasenjit.jnc at gmail.com>wrote: > Did you try using the acoustic sum rule and see whether the imaginary > frequencies are still there? Also can I have a look at the vc-relax output > file? > > Prasenjit > > > On 14 April 2014 11:10, ankit jain <ankitjain.me.iitk at gmail.com> wrote: > >> Hello Prasenjit, >> >> I am getting negative phonon frequencies for non-gamma points also. After >> structure optimization, the total force is 0.000020 Ry/au and pressure is >> -0.24 kbar. >> >> I am using ecutwfc of 100 Ry, which is converged with respect to the >> total energy of the system. I am not able to do convergence with respect to >> phonon modes as some modes have negative frequencies. Is there a way to do >> ecutwfc convergence for phonon modes without calculating phonon frequencies? >> >> I am atatching my pw.in, ph.in pw.out and ph.out files (which I forgot >> to attch in my original mail) here. >> >> Thanks, >> ANkit Jain >> >> >> On Mon, Apr 14, 2014 at 1:29 AM, Prasenjit Ghosh <prasenjit.jnc at gmail.com >> > wrote: >> >>> For which q-point are you getting the imaginary modes? If it is at >>> gamma, then it may be related to the instability in the structure, if it is >>> at other q-points, may be lattice parameter is not properly optimized. How >>> large is the stress you are getting at the end of the vc-relax calculation >>> (when the code recalculates the total energy, stress and forces with the >>> optimized lattice parameters & atomic positions). Also have you checked the >>> convergence of the phonon modes with ecutwfc because typically the phonons >>> converge much slowly with the size of the basis set than other parameters? >>> >>> With regards, >>> >>> Prasenjit >>> >>> >>> On 13 April 2014 21:55, Ankit <ankitjain.me.iitk at gmail.com> wrote: >>> >>>> Hello QE developers and users, >>>> >>>> I am trying to calculate phonon dispersion using ph.x and matdyn.x >>>> routines of QE and I am stuck woth negative frequencies for a while now. >>>> >>>> I have a 1-D chain of polyethylene which has 6 atoms in the unit-cell. >>>> The polymer chain is aligned in z-direction and I have added vacuum on >>>> x- and y- direction. I did the structure relaxation and I am getting >>>> total force after relaxtion as 0.000020, which I guess is sufficiently >>>> small. >>>> >>>> After relaxation, when I am trying to calculate phonon dispersion using >>>> ph.x and matdyn.x I am getting netting frequencies for some of the >>>> phonon modes. I searched online and on QE mailing list and tried couple >>>> of different things: >>>> >>>> 1. The negative frequencies are appearing for non-gamma point as well, >>>> so I guess its not a ASR issue. Anyway, I am using ASR but even after >>>> this I am getting negative frequencies. >>>> 2. I reduced my tr2_ph and conv_thr to 1.0d-14 and 1.0d-15 which I guess >>>> are very small as suggested on QE FAQs. >>>> 3. I checked phonon mode shapes for modes with negative frequencies and >>>> they do not look like rotational mode. >>>> 4. I tried changing number of k points from 6 to 12 in z direction but >>>> this is also not the cause. >>>> >>>> I am really stuck here and not sure about the cause of negative >>>> frequencies. With vc-relax, my structure is getting relaxed which I >>>> guess means my structure is stable. But then I am not sure why I am >>>> getting negative frequencies. >>>> >>>> I really appreciate any help or suggestions. >>>> >>>> Thanks, >>>> >>>> Ankit Jain >>>> IIT Indore, >>>> Indore, >>>> India >>>> >>>> >>>> _______________________________________________ >>>> Pw_forum mailing list >>>> Pw_forum at pwscf.org >>>> http://pwscf.org/mailman/listinfo/pw_forum >>>> >>> >>> >>> >>> -- >>> PRASENJIT GHOSH, >>> IISER Pune, >>> Dr. Homi Bhabha Road, Pashan >>> Pune, Maharashtra 411008, India >>> >>> Phone: +91 (20) 2590 8203 >>> Fax: +91 (20) 2589 9790 >>> >>> _______________________________________________ >>> 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 >> > > > > -- > PRASENJIT GHOSH, > IISER Pune, > Dr. Homi Bhabha Road, Pashan > Pune, Maharashtra 411008, India > > Phone: +91 (20) 2590 8203 > Fax: +91 (20) 2589 9790 > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://pwscf.org/mailman/listinfo/pw_forum > -------------- next part -------------- An HTML attachment was scrubbed... URL: http://pwscf.org/pipermail/pw_forum/attachments/20140414/5d160019/attachment.html
