[Pw_forum] Convergence of imaginary modes

Nicola Marzari nicola.marzari at materials.ox.ac.uk
Mon Jun 27 02:38:12 CEST 2011

On 6/26/11 7:02 PM, Brad Malone wrote:

> In previous lattice dynamical calculations, I've always been blessed
> with positive frequencies (at least those that can't be ASRed away) and
> I'm curious as to whether the more sensitive convergence of imaginary
> modes (and those that are close to being unstable) is a property that
> others have found in their calculations or whether it is something
> unusual with the system that I am studying.
> Thanks,
> Brad Malone
> UC Berkeley

Dear Brad,

if you have a Kohn anomaly it's not unusual to require that high
of a k-point sampling - comparable numbers are needed to converge
some of the optical modes in graphene/graphite (do check some of
the early papers of Mauri/Lazzeri).

I haven't heard of other cases where such high-sampling is needed
for phonons, unless your system was a metal, and you were not using
a smearing scheme (or way too small a smearing).


Prof Nicola Marzari    Department of Materials    University of Oxford
Chair of Materials Modelling  Director, Materials Modelling Laboratory
nicola.marzari at materials.ox.ac.uk     http://mml.materials.ox.ac.uk/NM

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