[Pw_forum] missing imaginary frequencies

[Paolo Giannozzi]

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
---
Paolo Giannozzi, Dept of Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222