Dear Stefano, Thanks a lot for your quick reply. Yes, both set of the frequencies are obtained after imposing ASR. All of frequencies are not negative. Two of the modes among total 5 modes (triply generate) are negative, which are not very unusual in a BaTiO3-type cubic perovskite.
regards, Saha S. K. On 5/11/11, Stefano de Gironcoli <degironc at sissa.it> wrote: > are you imposing acoustic sum rule ? this change slightly the frequencies . > isn't strange that your frequencies are all negative (imaginary) ? > is your system stable ? > stefano >
