[Pw_forum] negative frequencies....

[Paolo Giannozzi]

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 ( ω2≤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