Try to run the smallest system that yields strange results - without symmetry (just displace an atom a little bit) - in an older QE version (e.g. v4.3.2) - in the svn version. If imaginary frequencies disappear, please file a (usable) bug report (anything taking more than a few minutes is hardly usable). There is currently a known problem in the phonon code: http://qe-forge.org/gf/project/q-e/tracker/?action=TrackerItemEdit&tracker_item_id=106&start=0
P. On Fri, 2013-12-20 at 09:41 -0600, Jan Gryko wrote: > Dear QE users, > > The problem of negative frequencies from ph.x was discussed several times > on the PWSCF Forum. From what I read in the archives, the negative > frequencies > were due to too small number of k-points, or too small energy cutoff, or > non-zero forces acting on atoms (i.e. non-equilibrium geometry). > > Unfortunately, I am still getting negative frequencies for my system > (FCC cell, 34 Si atoms, semiconductor with ~0.7 eV gap). > The results were practically the same for: > > number of k-points: 1 (gamma point), 10, and 8x8x8 points (=29 k-points) > ecutwcf = 25, 30, and 55 Ry, and ecutrho = 4*ecutwfc > > In all cases, the geometry was optimized so the total force was 0.000002 > Ry/au, > and the max. force acting on an atom was 0.00000044 Ry/au. > Therefore, all atoms were in equilibrium positions, but yet about 10 first > phonon > frequencies were negative, and phonon dos (calculated with nq1=nq2=nq3=4) > was wrong. > > I noticed that ionic forces acting on atoms were large, ~0.7 Ry/atom, but > these > forces were compensated by opposite sign local forces. > > Is it possible that during perturbation calculations, these two opposite > forces are not > evaluated accurately, giving unstable system and negative frequencies? > > Any suggestions? Thank you very much. > > JG > > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://pwscf.org/mailman/listinfo/pw_forum -- Paolo Giannozzi, Dept. Chemistry&Physics&Environment, Univ. Udine, via delle Scienze 208, 33100 Udine, Italy Phone +39-0432-558216, fax +39-0432-558222
