On Thu, 2013-10-10 at 10:57 +0200, Uri Argaman wrote: > 1. unstable structure should lead to imaginary frequency not a > negative one. > > so how come it become negative in the phonon dispersion? is it > related to the square of it?
from the FAQ: 7.3 Why do I get negative phonon frequencies? ?Negative? frequencies actually are ?imaginary? frequencies ( ?2?0 ). If these occur for acoustic frequencies at Gamma point, or for rotational modes of a molecule, see above. In all other cases: it depends. It may be a problem of bad convergence (see above), or it may signal a real instability. and above: 7.2 I am not getting zero acoustic mode frequencies, why? Because the Acoustic Sum Rule (ASR), i.e. the translational invariance, is violated in approximated calculations. In PW calculations, the main and most irreducible violation comes from the discreteness of the FFT grid. There may be other reasons, though, notably insufficient convergence: ?Recently I found that the parameters tr2_ph for the phonons and conv_thr for the ground state can affect the quality of the phonon calculation, especially the ?vanishing? frequencies for molecules.? (Info from Katalyn Gaal-Nagy). Anyway: if the nonzero frequencies are small, you can impose the ASR to the dynamical matrix, usually with excellent results. [...] > 2. how can we distinguish between unstable structure and problems with > an convergence ones? see above: in truly unstable structures "negative" frequencies will not be fixed by imposing ASR P. -- Paolo Giannozzi, Dept. Chemistry&Physics&Environment, Univ. Udine, via delle Scienze 208, 33100 Udine, Italy Phone +39-0432-558216, fax +39-0432-558222
