[Pw_forum] Negative frequencies from ph.x
paolo.giannozzi at uniud.it
Sat Dec 21 16:36:43 CET 2013
Try to run the smallest system that yields strange results
- without symmetry (just displace an atom a little bit)
- in an older QE version (e.g. v4.3.2)
- in the svn version.
If imaginary frequencies disappear, please file a (usable)
bug report (anything taking more than a few minutes is hardly
usable). There is currently a known problem in the phonon code:
On Fri, 2013-12-20 at 09:41 -0600, Jan Gryko wrote:
> Dear QE users,
> The problem of negative frequencies from ph.x was discussed several times
> on the PWSCF Forum. From what I read in the archives, the negative frequencies
> were due to too small number of k-points, or too small energy cutoff, or
> non-zero forces acting on atoms (i.e. non-equilibrium geometry).
> Unfortunately, I am still getting negative frequencies for my system
> (FCC cell, 34 Si atoms, semiconductor with ~0.7 eV gap).
> The results were practically the same for:
> number of k-points: 1 (gamma point), 10, and 8x8x8 points (=29 k-points)
> ecutwcf = 25, 30, and 55 Ry, and ecutrho = 4*ecutwfc
> In all cases, the geometry was optimized so the total force was 0.000002 Ry/au,
> and the max. force acting on an atom was 0.00000044 Ry/au.
> Therefore, all atoms were in equilibrium positions, but yet about 10 first phonon
> frequencies were negative, and phonon dos (calculated with nq1=nq2=nq3=4) was wrong.
> I noticed that ionic forces acting on atoms were large, ~0.7 Ry/atom, but these
> forces were compensated by opposite sign local forces.
> Is it possible that during perturbation calculations, these two opposite forces are not
> evaluated accurately, giving unstable system and negative frequencies?
> Any suggestions? Thank you very much.
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Paolo Giannozzi, Dept. Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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