Dear Andrei Thanks a lot for your answer; unfortunately you've confirmed my fears on the real meaning of negative frequencies.
The forces level of the configuration is not larger than 0.02 eV/Ang, which is half the default (zero) value. I thus expected to be on the safe side, but as you suggest this is apparently not the case ;( . Perhaps the default displacement, 0.04 Bohr, is also a bit too large ?. Anyway, I'll redo the calculations playing with the MeshCutoff then. Thanks again, Roberto On Wed, 15 Apr 2009, apost...@uni-osnabrueck.de wrote:
Dear Roberto, the frequencies appearing in VIBRA as negative are in fact imaginary. The reason for their appearance is too bad forces (big numerical noise in forces because of unsufficient MeshCutoff) and/or calculation of phonos too far from equilibrium (unsufficient structure relaxation prior to phonon calculation). In a calculation of Gamma-phonon you'd have three acoustic frequencies which must be close enough to zero; but numerically thay may deviate a bit by ~0.1; if some of them are negative, this is no problem. Otherwise, big negative (imaginary) frequencies mean a problem. Usually you should check MeshCutoff, check relaxation and repeat the phonon calculation. Sincerely, Andrei Postnikov
