[QE-users] Negative phonon frequencies

[sally issa]

Hi everyone,
I am working with MgB₂ in the following structure:

Mg   0.00000000  0.00000000  0.00000000
B    0.33333333  0.66666667  0.50000000
B    0.66666667  0.33333333  0.50000000

I first performed relaxation calculations to determine the optimal parameters (ecutwfc, celldm, K-mesh, etc.). My calculation has converged well using vc-relax.
Now, I am trying to calculate phonons using different q-mesh grids. However, with an 8×8×8 grid, I obtain negative frequencies around the Gamma point (see attached figure).
For that, I have run the following input files:

  *   scf.in
  *   ph.in
  *   q2r.in
  *   matdyn.in

I am using a NCPP/PBE pseudopotential.
Do you have any ideas about the possible causes of this issue or suggestions to improve convergence and avoid these negative frequencies?
Thanks in advance for your help!
Best Regards,
S.Issa

[cid:b8e0ae5e-927e-4059-8eca-77304103a74b]

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