Dear Paolo and Serge, thank you very much for your reply, that's it, in order to make the dispersion calculation I used Z=0,0,1/2 and used the same for the individual k-point calculation of my tetragonal system. After trying with z=0,0,c/a I got the correct results. Best Regards, andrea
Serge Nakhmanson wrote: > 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 >> > > >
