Sergio Yanuen Rodriguez wrote: >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 > > > Dear Sergio,
one should do a volume optimization before computing the Hellmann-Feynman forces. If the negative frequencies phonon frequencies at gamma are not too small (nearly zero) they may be due to some numerical errors. You can then impose the translation-rotational invariances (in create/hellmann-feynman f./trans.rotInv.). From my experience a 2x2x2 supercell should be ok. Do you use LDA or GGA? Did you displace the atoms in positive and negative direction ? If you did not use optimized lattice parameter and did not displace in positive and negative direction, then I would try first to optimize and then displace in positive and negative direction. I hope this helps Marco >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 > > -- ---------------------------------------------------------- Dr. Marco Schowalter Bureau M.4180 IFP - University Bremen Otto-Hahn-Allee 1 28359 Bremen GERMANY Tel: ++49 (0)421 218 3566 Fax: ++49 (0)421 218 7381 Email: schowalter at ifp.uni-bremen.de ---------------------------------------------------------- Please avoid sending me MS-file attachments. See http://www.gnu.org/philosophy/no-word-attachments.html Thank you. ----------------------------------------------------------
