Dear Gabriele, yes, I changed the volume, it's less then the relaxed value. this way TO phonon is stable at gamma point. the right values for TA branch should go to 0. Alexander
Le 20 ao?t 2014 ? 10:45, "Gabriele Sclauzero" <gabriele.sclauzero at mat.ethz.ch<mailto:gabriele.sclauzero at mat.ethz.ch>> a ?crit : Dear Alexander, On 08/19/2014 05:08 PM, Kvasov Alexander wrote: Dear developers, I study flexoelectric effect which can be seen as a repulsion of TA and TO branches in the vicinity of gamma point. I calculate phonon dispersion of cubic SrTiO3. It?s essential for me to have adequate picture of phonons in the vicinity of gamma point (at least for q>0.05). The picture shows TA and TO branches single q-point (st.ph.single.in) calculations (grey dots) and dispersion (st.ph.disp.in) for 8x8x8 mesh. With used parameters ph.x fails to give right values for small values of q-vector. What are the "right values" you were expecting? Probably you know this already, but within PBE (probably also LDA) SrTiO3 displays a ferroelectric instability at the theoretical lattice constant, which triggers the imaginary frequency at Gamma. This is an artifact of the approximate functional. You might get rid of that by changing the volume (I don't remember in which direction, hopefully toward the experimental value...), or using some other functional. HTH GS I guess it?s due to numerical errors of the code. What are the main reasons for this numerical error when we go close to gamma point. Which parameters should I adjust to improve the situation? Attached, all relevant inputs and outputs Sincerely yours, Alexander Kvasov Ceramics Laboratory / THEOS (Theory and Simulation of Materials) Laboratory ?cole Polytechnique F?d?rale de Lausanne (EPFL) Phone: +41 21 693 5805 Address: EPFL STI IMX LC MXD 220 (B?timent MXD) Station 12 1015 Lausanne, Switzerland -- Dr. Gabriele Sclauzero ETH Zurich Materials Theory HIT G 43.2 Wolfgang-Pauli-Str. 27 8093 Z?rich, Switzerland Phone +41 44 633 94 10 Fax +41 44 633 14 59 gabriele.sclauzero at mat.ethz.ch<mailto:gabriele.sclauzero at mat.ethz.ch> http://www.theory.mat.ethz.ch/people/postdocs/gsclauze _______________________________________________ Pw_forum mailing list Pw_forum at pwscf.org<mailto:Pw_forum at pwscf.org> http://pwscf.org/mailman/listinfo/pw_forum -------------- next part -------------- An HTML attachment was scrubbed... URL: http://pwscf.org/pipermail/pw_forum/attachments/20140820/83a3ec97/attachment.html
