[Pw_forum] keep getting negative frequencies in phonon calculation

[Khalid Ibne Masood Khalid]

Dear Sir,
Thank you very much for your kind help.


 Khalid Ibne Masood
 M.Sc student
 BUET


On Sat, Aug 8, 2015 at 2:04 PM, David Foster <davidfoster751 at yahoo.com>
wrote:

> I have encountered such a problem for my studies. Taking care of following
> points may help you:
>
> 1- see the stress and pressure for your crystal. its force should be very
> low.
> 2- use large cutoff for wfn and rho, even much larger than the converged
> values against energy/atom
> or force/atom (1mRy/atom).
> 3- use very dense k-point grid in scf
> 4- use very low convergence threshold for scf (10^(-10) for example), and
> for phonon (10^(-14))
> 5- if after dynmat.x, the acoustic phonons are near to 1cm-1, it is good.
> 6- taking care of spin-magnetization.
>
> Regards
>
> David Foster
>
> Ph.D. Student of Chemistry
>
> --------------------------------------------
> On Sat, 8/8/15, Khalid Ibne Masood Khalid <kimu206 at gmail.com> wrote:
>
>  Subject: [Pw_forum] keep getting negative frequencies in phonon
> calculation
>  To: "PWSCF Forum" <pw_forum at pwscf.org>
>  Date: Saturday, August 8, 2015, 12:02 AM
>
>  Dear
>  Researchers,
>  I think this is an old problem, but I require your
>  advice about how to solve this problem. I am trying to
>  calculate the phonon dispersion curve of a 2D insulator
>  material, and followed example 02 of phonon program. In the
>  ph.x input file I have used:
>
>    tr2_ph=1.0d-12,
>    ldisp=.true.,
>    nq1=2, nq2=2, nq3=1
>
>  [increasing nq1 and nq2 will be more complicated]
>
>  I have tried almost all types of asr, zero-dim,
>  crystal and simple, but yet I am getting negative
>  frequencies in the phonon dispersion curve.
>
>  Now Sir, what should I do to correct this problem.
>  Should ldisp = .false. solve the problem?
>
>  Thank you.
>
>  Khalid Ibne Masood
>  M.Sc student
>  BUET
>
>
>
>
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>
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