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 *********************************************************
