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

[Paolo Giannozzi]

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 août 2014 à 10:45, "Gabriele Sclauzero"
> <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
> > http://www.theory.mat.ethz.ch/people/postdocs/gsclauze
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-- 
Paolo Giannozzi, Dept. Chemistry&Physics&Environment, 
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222