Dear Thierry Tsafack and QE users, I actually have similar problems as Theirry. I am recently trying to calculate phonon dispersion relation for two dimensional materials, but always got negative frequencies.
First, I am pretty sure my structure is fully relaxed. It is also stable based on the literature report (they use VASP for calculation). I used norm-conserving type of pseudopotential. The negative frequency appears near the zone-center. At zone center the frequency is exactly zero. I've tried to resolve the problem using the following attempts: 1. Change different ASR 2. Increase ecutrho to from 240 to 480 Ry (8 times ecut) 3. Increase ecutwfc from 45 to 90 4. Increase k-point mesh size from 16*16*1 to 32*32*1 5. Increase tr2_ph from 1e-18 to 1e-20 6. Increase conv_thr from 1e-8 to 1e-12 Since the calculations are pretty time consuming, it is difficult to try even smaller convergence threshold. I really hope someone can help with this issue. Thanks, Hua Bao On Sat, Nov 2, 2013 at 4:03 AM, Thierry Tsafack <tr.thierry at rice.edu> wrote: > > > Dear fellow QE users, > > I hope this email finds you well. > > Could you please help me fix the issues below? > > 1-) The executable ph.x requires the knowledge of nq1 nq2 nq3 to > determine the density of the Monkhorst-Pack grid, i.e, number of > points and their coordinates. What is the linkage between nq1 nq2 nq3 > and the number of points in the grid? > > Everything else being equal, 3x3x1 yielded 5 points, 4x4x1 yielded 10 > points, 5x5x1 yielded 13 points, 6x6x1 yielded 20 points, 7x7x1 > yielded 25 points. > > For benchmark purposes, how do I know how many points nq1 x nq2 x 1 > will produce without doing the calculations? > > 2-) How to go about a situation in which ph.x produces w(q_i)>0 for > all i in the above grid, but q2r.x and matdyn.x produce some w(q_i)<0, > for all choices of the ASR or ZASR? Assume the geometry relaxation > was done as well as the scf calculation and you know that the > structure is stable. > > 3-) Recover =.true. in QE-4.3 always fails saying:"from checkallsym : > error". How to go around it? > > Thanks in advance for your clues, if your answer necessitates further > details, let me know. > > > ~Thierry T. > MEMS Rice University, Houston, TX > > > _______________________________________________ > 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/20131103/ce8ee389/attachment.html
