On Wed, 2014-08-20 at 09:13 +0000, Kvasov Alexander wrote: > the right values for TA branch should go to 0.
Acoustic modes NEVER go to zero frequency at q=0, in practice: http://www.quantum-espresso.org/faq/phonons/#7.2 I am not sure if and how the violation of the acoustic sum rule affects frequencies at small q, but I wouldn't be surprised if some effect is visible also at q!=0. Paolo > Alexander > > Le 20 ao?t 2014 ? 10:45, "Gabriele Sclauzero" > <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 > > http://www.theory.mat.ethz.ch/people/postdocs/gsclauze > > _______________________________________________ > > Pw_forum mailing list > > Pw_forum at pwscf.org > > http://pwscf.org/mailman/listinfo/pw_forum > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://pwscf.org/mailman/listinfo/pw_forum -- Paolo Giannozzi, Dept. Chemistry&Physics&Environment, Univ. Udine, via delle Scienze 208, 33100 Udine, Italy Phone +39-0432-558216, fax +39-0432-558222
