what is kind of pseudopotential? How much k-point?
wishes sanjeev On Sun, Apr 13, 2014 at 9:25 AM, 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 > -- With Best Regards, ------------------------------------ Dr. Sanjeev Kumar Gupta Fulbright Post-Doctoral Scholar Dept. of Physics Michigan Technological University 1400 Townsend Drive, Houghton MI 49931, USA ------------------------------------ -------------- next part -------------- An HTML attachment was scrubbed... URL: http://pwscf.org/pipermail/pw_forum/attachments/20140413/b4a93799/attachment.html