> 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 ;( .
Dear Roberto: what do you mean by "The forces level of the configuration is not larger than 0.02 eV/Ang, which is half the default (zero) value" ? That you relaxed your system well before doing the phonons? That's OK, but you should really check MeshCutoff first. What you should care about is that the sum of forces over all atoms remains small enough (typically within +/- 0.1 eV/Ang), and this independently on uniform displacement of all atoms. Making this eggbox test fixes your necessary value of MeshCutoff. > Perhaps the default displacement, 0.04 Bohr, is also a bit too > large ?. This is about how anharmonic the vibrations are. Usually this parameter does not change much, and in any case it cannot be resonsible for imaginary frequencies. Best regards Andrei
