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 > -------------- next part -------------- An HTML attachment was scrubbed... URL: http://pwscf.org/pipermail/pw_forum/attachments/20140414/04a29159/attachment.html -------------- next part -------------- A non-text attachment was scrubbed... Name: ph.in Type: application/octet-stream Size: 185 bytes Desc: not available Url : http://pwscf.org/pipermail/pw_forum/attachments/20140414/04a29159/attachment.obj -------------- next part -------------- A non-text attachment was scrubbed... Name: ph.out Type: application/octet-stream Size: 123825 bytes Desc: not available Url : http://pwscf.org/pipermail/pw_forum/attachments/20140414/04a29159/attachment-0001.obj -------------- next part -------------- A non-text attachment was scrubbed... Name: pw.in Type: application/octet-stream Size: 767 bytes Desc: not available Url : http://pwscf.org/pipermail/pw_forum/attachments/20140414/04a29159/attachment-0002.obj -------------- next part -------------- A non-text attachment was scrubbed... Name: pw.out Type: application/octet-stream Size: 14629 bytes Desc: not available Url : http://pwscf.org/pipermail/pw_forum/attachments/20140414/04a29159/attachment-0003.obj