[QE-users] Optical branches are not continuous at gamma point in phonon dispersion

[Changpeng Lin]

Hi,

Thanks for your help. According to the suggestions, I have tested the
convergence of dielectric constants and effective charges with respect to k
mesh and they in fact converged with the size of k mesh.

As Prof. Marzari indicated, the phonon dispersion including LO-TO splitting
might have the discontinuity becase it depends how q vector appoaching
gamma point. It should be corrent. I also found some example of this kind
unusal phonon dispersion in literature.

Thanks,
Changpeng

Marzari Nicola <nicola.marzari at epfl.ch> 于2018年11月14日周三 下午4:32写道：

>
>
> Note you can converge to different limits depending on direction, so a
> discontinuity in a plot where the direction changes angle is possible.
> Nicola
>
> Sent from a tiny keyboard... Contact info:
> http://theossrv1.epfl.ch/Main/Contact
>
> On 14 Nov 2018, at 09:29, Lorenzo Paulatto <paulatz at gmail.com> wrote:
>
> >> I found that optical branches are not continuous at gamma point (See
> figure below). I guess this is becasue the wrong LO-TO spiliting relation I
> got. How can I check it?
> >
> > It is definitely because of LO-TO splitting, if it is wrong or not, well
> you can check how it converges with k-points. Dielectric constants and
> effective charges can be VERY slow to converge with k-points. Just repeat
> the phonon calculation at Gamma, no need to redo all the grid each time.
> >
> > Or if you think the should be no LO-TO splitting at all (typically
> because of doping), just edit the first dyn matrix file (the one at gamma,
> i.e. dyn1) and remove all the section about epsilon and zeu, from
> >   "Dielectric Tensor:"
> > included, down to
> >   "Diagonalizing the dynamical matrix"
> > excluded.
> >
> > hth
> >
> >
> > --
> > Lorenzo Paulatto - Paris
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