It looks like a problem with a symmetry having an associated fractional translation. It would be interesting to know whether this problem is still present in the svn version, and if not, a smaller test showing the same problem
P. On Mon, 2013-11-25 at 23:05 +0800, Tao Sun wrote: > Dear users, > > > When we calculate the phonon spectrum of a rather obscure mineral, > K2Si4O9, at high pressures, we encounter a strange problem. > > > The unit cell contains 60 atoms, for which the code found two Sym. > ops. One is the identical operator, the other is 180 deg rotation - > cart. axis [0,0,1] plus (0,0,0.5) fractional translation. Using the > default 'nosym = .false.' in the scf input, we found large imaginary > frequencies at q=(0,0,1/2), > > omega( 1) = -89.679305 [THz] = -2991.379628 [cm-1] > omega( 2) = -89.679305 [THz] = -2991.379628 [cm-1] > omega( 3) = -88.598203 [THz] = -2955.317953 [cm-1] > omega( 4) = -88.598203 [THz] = -2955.317953 [cm-1] > omega( 5) = 2.408244 [THz] = 80.330387 [cm-1] > omega( 6) = 2.408244 [THz] = 80.330387 [cm-1] > omega( 7) = 2.465075 [THz] = 82.226043 [cm-1] > omega( 8) = 2.465075 [THz] = 82.226043 [cm-1] ... > > > Searching more stable structures along the presumed 'unstable' modes > was in vain, which led us to suspect the above imaginary frequencies > are artifacts. Indeed, by setting the 'nosym = .true.', we found > > omega( 1) = 2.289272 [THz] = 76.361882 [cm-1] > omega( 2) = 2.289349 [THz] = 76.364467 [cm-1] > omega( 3) = 2.373282 [THz] = 79.164179 [cm-1] > omega( 4) = 2.373320 [THz] = 79.165434 [cm-1] > omega( 5) = 2.668095 [THz] = 88.998078 [cm-1] > omega( 6) = 2.668166 [THz] = 89.000448 [cm-1] > omega( 7) = 2.867070 [THz] = 95.635151 [cm-1] > omega( 8) = 2.867215 [THz] = 95.639985 [cm-1]... > > > This problem is independent of the pseudopotentials used, and appears > in pwscf 4.3.2 as well as the patched 5.0.3. > > > The relevant input and output files are attached below. To speed up > reproducing the error, the k-point mesh was trimmed to gamma point > only. > > > Thank you. > > Tao Sun > > College of Earth Sciences > > University of Chinese Academy of Sciences > > _______________________________________________ > 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
