[Pw_forum] Negative frequence at Gamma point and other kpoints

[Mohammed Ghadiyali]

Hi,


It might be because you have not geometry optimized your system completely.

I recommend you to run a relax calculation with following parameters changed, they might seem overkill but they work:


in CONTROL

  etot_conv_thr         =       1.0D-6,
  forc_conv_thr         =       1.0D-5,
in SYSTEM
  nosym                 =       .TRUE.,
in ELECTRON
  diagonalization       =       'cg',
  conv_thr              =       1D-8,

With a dense k-point grid, however as the symmetry is broken the k-point grid might be dense - you need to check.
Then a vc-relax run with same parameters (and update atomic positions), but set cell_dofree as per your requirement.
You also need to check your structure after the geometry optimization, there might be a change in angle from say 90 degrees to 89.9 degrees, this will also give you negative or imaginary frequences.

I also recommend you to get the crystal structure data form: materialsproject.org
They are generally optimized or with very fewer forces.

Regards,
Ghadiyali Mohammed Kader
Dept. of Physics,
University of Mumbai.

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