[Pw_forum] Negative frequencies from ph.x

[Paolo Giannozzi]

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:
http://qe-forge.org/gf/project/q-e/tracker/?action=TrackerItemEdit&tracker_item_id=106&start=0

P.

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.
> 
>  JG
> 
>  
>   
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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