[Pw_forum] missing imaginary frequencies

[Andrea Salguero]

Dear Eyvaz,
thank you for your reply, the problem is not in the phonon dispersion 
but in the calculation of the frequency at Z point, other calculations 
show there is an instability at that point, which is observed in the 
phonon dispersion I have obtained with PWscf code as well. However, I 
don't get imaginary frequencies when calculating the frequencies only at 
that point.
Best Regards,
Andrea
 
Eyvaz Isaev wrote:
> Dear Andrea,
>
> Have you tried using more q-points? Can you send me the picture to have a look 
> at?
>
> Bests,
> Eyvaz.
> -------------------------------------------------------------------
> Prof. Eyvaz Isaev, 
> Department of Physics, Chemistry, and Biology (IFM), Linkoping University, 
> Sweden 
>
> Theoretical Physics Department, Moscow State Institute of Steel & Alloys, 
> Russia, 
>
> isaev at ifm.liu.se, eyvaz_isaev at yahoo.com
>
>
>
> ----- Original Message ----
> From: Andrea Salguero <ls538 at cornell.edu>
> To: "pw_forum at pwscf.org" <pw_forum at pwscf.org>
> Sent: Sat, January 15, 2011 1:20:18 AM
> Subject: [Pw_forum] missing imaginary frequencies
>
> Dear Pwscf community:
>
> I have calculated the phonon frequencies of a metallic system along different 
> high symmetry points in the BZ and also the phonon dispersion of it. I see two 
> instabilities at z-point in the dispersion plot but I obtain no 
> imaginary/negative frequencies when I calculate only for that point. I used the 
> same parameters in the input files for both calculations and I followed the 
> procedure from the examples directory of PWSCF(example02) for the calculation of 
> the frequencies at Z point:
> 1) a scf calculation
> 2) then the calculation of the dynamical matrix at z-point (0,0,0.5), my input 
> file:
>
> &inputph
>   tr2_ph=1.0d-14,
>   amass(1)=183.84d0,
>   amass(2)=15.9994d0,
>   prefix='case',
>   outdir = '',
>   fildyn='case.dynZ',
> /
> 0.0 0.0 0.5
>
> what am I doing wrong or what am I missing?
>
> Any help would be appreciated,
>
> Best Regards
> Andrea
>
>
> ********************************************************
> Andrea Salguero
> School of Applied and Engineering Physics
> Cornell University, Ithaca NY
> ********************************************************
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