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 > Hello everybody, > > Perhaps some of you can give a quick answer to the following question, > I'm computing the phonon spectra at the \gamma point for a rather > big unit cell (some 50 atoms) using the VIBRA package. > > The point is that about half of the frequencies are coming out > NEGATIVE, though not IMAGINARY. > This is somewhat unexpected to me, though not impossible because > after all the solutions are combinations of $ \exp(\pm \omega t) $. > > I wonder then if those negative numbers really mean imaginary, or > in other words, should I worry about the result ?. > > Thanks a lot. > > Cheers, > > Roberto
