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

[Gabriele Sclauzero]

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
http://www.theory.mat.ethz.ch/people/postdocs/gsclauze

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20140820/953f0cdd/attachment.html>