[Pw_forum] keep getting negative frequencies in phonon calculation

[David Foster]

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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