[Pw_forum] ph - computational errors in vicinity of gamma point

[Kvasov Alexander]

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 août 2014 à 10:45, "Gabriele Sclauzero" <gabriele.sclauzero at mat.ethz.ch<mailto:gabriele.sclauzero at mat.ethz.ch>> a écrit :

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
École Polytechnique Fédérale de Lausanne (EPFL)

Phone: +41 21 693 5805

Address:
EPFL STI IMX LC
MXD 220 (Bâtiment MXD) Station 12
1015 Lausanne, Switzerland




--
Dr. Gabriele Sclauzero
ETH Zurich
Materials Theory
HIT G 43.2
Wolfgang-Pauli-Str. 27
8093 Zürich, 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

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