[Pw_forum] Re: convergence of phonon

[Stefano de Gironcoli]

There can be many reasons for appearance of negative (actually 
immaginary) frequencies
in a stable system (insufficient convergence, poor k-point sampling, bad 
pseudopotentials,
bug in the code...).
But it is also possible that the calculation is telling you something 
about your system.
Your forces are zero, most of them for symmetry reasons (judging from 
the fact that they are exacly zero!!!).
It is possible that your system is not in stable equilibrium but in a 
saddle point (that can be
physically relevant or due to poor k-point sampling, insufficient 
cut-off, bad xc-functional,
bad pseudopotential, ...)
Your negative fequencies are two-fold degenerate that means for sure 
that they do not belong
to the totally symmetric irreducible representation of your crystal 
symmtry (the one that is
preserved during relaxation).
Try breaking the symmetry of your system moving the atoms along one ove 
the unstable eigenvectors
or just add some random displacements to your coordinates and relax again .
Does the system go back to the original position ? if this happens there 
is a problem in the phonon calculation.
Or the system goes somewhere else ? If this happens your original 
positions actually  correspond
to a saddle point and the phonon code spotted it.

I'm suspicious about your degauss, it looks very small to me.
I would rather use a larger degauss and m-p or m-v smearing
Make a plot of your DOS broadening it with  FD with degauss=0.002
If your sampling is sufficiently dense the DOS will be smooth, otherwise 
it will  wildy oscillate.

best regards,

   Stefano de Gironcoli

Eduardo Ariel Menendez P wrote:

>Hello phonon community,
>Thanks to nicola marzari for his advice on the parameters for phonon
>calculation.
>However, I have tested the convergence of the phonon
>frequencies against the wavefunction cutoff, and I do not find convegence.
>I am surprised that I obtain very large negative frequencies like
>     omega( 1) =      -4.995406 [THz] =    -166.629919 [cm-1]
>     omega( 2) =      -4.995406 [THz] =    -166.629919 [cm-1]
>     omega( 3) =      -4.474172 [THz] =    -149.243320 [cm-1]
>     omega( 4) =      -4.474172 [THz] =    -149.243320 [cm-1]
>     omega( 5) =      -2.511886 [THz] =     -83.788069 [cm-1]
>     omega( 6) =      -2.511886 [THz] =     -83.788069 [cm-1]
>     omega( 7) =       0.734339 [THz] =      24.495069 [cm-1]
>     omega( 8) =       1.858145 [THz] =      61.981460 [cm-1]
>  etc, up to omega(36)
>  
>