[Pw_forum] missing imaginary frequencies

[Serge Nakhmanson]

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


-- 
*********************************************************
  Serge M. Nakhmanson               phone: (630) 252-5205
  Assistant Scientist                 fax: (630) 252-4798
  MSD-212, Rm. C-224
  Argonne National Laboratory
  9700 S. Cass Ave.
  Argonne, IL 60439
*********************************************************