[Pw_forum] missing imaginary frequencies

[Eyvaz Isaev]

Well, let me add a few words to this issue.

>This problem does not come up in phonon-dispersion calculations, since
>there the k-point grid is generated automatically (and correctly, as
>one would hope), so that Z is always what it should be.


If one uses PlotPhon utility, yes, of course, the c/a ratio is properly 
accounted for  

for phonons along G-Z.
So, (0,0,0.5) is not a real q-point (for c/a=1 only), it should be 
(0,0,0.5*a/c). 

In my practice, I never got such kind surprise, except a case when I got large 
imaginary frequencies, but restarting the job fixed this problem, that, most 
likely, 

was due to a system error.

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: Serge Nakhmanson <nakhmanson at anl.gov>
To: PWSCF Forum <pw_forum at pwscf.org>
Sent: Tue, January 18, 2011 11:12:08 PM
Subject: Re: [Pw_forum] missing imaginary frequencies

My advice would be to recheck, for an individual k-point calculation,
that the point you think is Z is actually Z in QE's units, which are
*not* crystal units. E.g., for a tetragonal P4mm system with c/a != 1,
Z will not be [0,0,1/2], but something else, depending on the c/a ratio.

This problem does not come up in phonon-dispersion calculations, since
there the k-point grid is generated automatically (and correctly, as
one would hope), so that Z is always what it should be.

S.

Paolo Giannozzi wrote:
> On Jan 17, 2011, at 20:02 , Andrea Salguero wrote:
> 
>> 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.
> 
> very strange. If the Z point is in the wavevector grid used to calculate
> interatomic force constants in real space, you should get the same
> results from the single and the complete calculation. If not, there can
> be a small difference, but nothing more than a small difference, if
> everything is properly done
> 
> Paolo


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