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

[Stefano Baroni]

I beg to differ. Whatever the specific reason may be in this case, negative frequencies do *not* (necessarily) indicate incomplete relaxation, but the fact that the nearest equilibrium structure is unstable (i.e. it is a saddle point, rather than a minimum). If the current configuration is close enough to a minimum, the computed frequencies should be positive no matter whether you are at an equilibrium configuration or not. SB

— 
Stefano Baroni - Trieste —  http://stefano.baroni.me <http://stefano.baroni.me/> 




> On 9 Dec 2020, at 12:39, Tamas Karpati <tkarpati at gmail.com> wrote:
> 
> 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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