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 -------------- next part -------------- An HTML attachment was scrubbed... URL: http://pwscf.org/pipermail/pw_forum/attachments/20140414/22e01e75/attachment.html