This is NaCl, thus no need (no possibility) to optimize positions. Of course the volume affects the phonon frequencies, but unless you have a very unreasonable volume (mixed Ang and bohr ????) the NaCl structure should be stable and thus imaginary frequencies should not appear.
I'd expect you made an error during init_phonon. Remember, you must not accept changes by sgroup (when it wants to change the lattice), but have to find the proper symmetry by nn. Maybe try first a 1x1x1 supercell. Of course in this case the supercell is too small, but still at Gamma and X (?) the frequencies should be exact. Sergio Yanuen Rodriguez schrieb: > Dear Wien2k users > > I am trying to do phonon calculations for NaCl because this system is > already studied and after this apply this method to something more > complicated, I generate the case.d45 with PHONON for a 2X2X2 supercell, > then I transfer this file to Wien 2k and calculate the forces, but when I > run analyse_phonon the sum of this forces are not small, and the results I > get do not seem alright. I compared this results with another ones and > they do not agree specially for the acoustic modes. Since I am getting > negative frequencies at Gamma where they must have 3 (nearly) zero > frequencies. > > Does any one know which parameter is important to adjust to make the sum > of the forces small? or do I have to do a volume optimization before > calculating the forces? Or any suggestions > > I really appreciate any help. > > Sergio Y. Rodriguez > Physics Department > Texas A&M University > > > _______________________________________________ > Wien mailing list > Wien at zeus.theochem.tuwien.ac.at > http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien -- P.Blaha -------------------------------------------------------------------------- Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna Phone: +43-1-58801-15671 FAX: +43-1-58801-15698 Email: blaha at theochem.tuwien.ac.at WWW: http://info.tuwien.ac.at/theochem/ --------------------------------------------------------------------------