[QE-users] Either NEGATIVE FREQUENCY or CONVERGENCE problem in PHONON

[Tamas Karpati]

apart from the convergence issues negative frequencies from converged
calculations suggest
that your structure -as opposed to your standpoint- is not stable or
the geom. optimization was not
complete (many negative freqs. may indicate not being at the exact
energy minimum).
you can reoptimize (both cell and ions, ie. via vc-relax) the geometry
and next redo the phonon
sim. to check it.

On Wed, Dec 9, 2020 at 11:47 AM Sohail Ahmad via users
<users at lists.quantum-espresso.org> wrote:
>
> Dear QE experts
> I am trying to calculate the phonon of NiCrSi using QE 6.5.
>
> Either i get negative frequency or it does not converge
> I tried with different values of convergence criteria, q mesh, alpha_mix in ph.in and k mesh in scf.in
>
> tr2_ph = 1.0d-14, or
> tr2_ph = 1.0d-12, or
> tr2_ph = 1.0d-16,
>
> nq1 = 4, nq2 = 4, nq3 = 4, or
> nq1 = 3, nq2 =3, nq3 = 3, or
>   nq1 = 2, nq2 = 2, nq3 =2,
>
> alpha_mix(1) = 0.7, or
>   alpha_mix(1) = 0.3,
>
> K_POINTS
> 12 12 12 or
> 16 16 16
>
> What could be the possible reason of getting negative frequency if the structure is considered as stable ? Appreciate if someone point out the possible mistakes
> Best Wishes
> ---------------------------------------------------------
>
> #phonon of NiCrSi structure
> &inputph
> prefix = 'NiCrSi',
> tr2_ph = 1.0d-14,
> ldisp = .true.,
> nq1 = 4, nq2 = 4, nq3 = 4,
> amass(1) = 51.99,
> amass(2) = 28.08,
> amass(3) = 58.69,
>   alpha_mix(1) = 0.7,
> outdir = './OUT',
> fildyn = 'NiCrSi.dyn',
> /
> &control
>   calculation = 'scf',
>   restart_mode = 'from_scratch',
>   prefix = 'NiCrSi',
>   pseudo_dir = '/home/sohail/pseudo',
>   outdir = './OUT',
> /
> &system
> ibrav = 2, a = 5.4559000675, b = 5.4559000675, c = 5.4559000675,
> cosAB = 0.0, cosBC = 0.0, cosAC = 0.0,
>   nat = 3, ntyp = 3,
>   ecutwfc = 75,
>   ecutrho = 300,
>   nbnd = 25,
>   occupations = 'smearing', smearing = 'gaussian', degauss = 0.001,
> /
> &electrons
>   mixing_beta = 0.3,
>   conv_thr = 1.0d-9,
> /
> ATOMIC_SPECIES
> Cr  51.99  Cr.pbe-sp-van.UPF
> Si  28.08  Si.pbe-n-van.UPF
> Ni  58.69  Ni.pbe-nd-rrkjus.UPF
> ATOMIC_POSITIONS  {alat}
> Cr  0.00000000  0.00000000  0.00000000
> Si  0.50000000  0.50000000  0.50000000
> Ni  0.25000000  0.25000000  0.25000000
> K_POINTS AUTOMATIC
> 12 12 12 0 0 0
>
> ---------------------------------------------------------------------
>
> Sohail Ahmad
>
> Department of Physics
>
> Faculty of Science, P. O. Box - 9004
>
> King Khalid University
>
> Abha, Saudi Arabia
>
> --------------------------------------------------------------------
>
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