[QE-users] phonon calculations on polar slabs - imaginary modes

Ilya Ryabinkin igryabinkin at gmail.com
Fri Apr 6 16:05:01 CEST 2018

A simple question: did you relax your slabs?

On Fri, Apr 6, 2018 at 9:57 AM, Christoph Wolf <wolf.christoph at qns.science>

> Dear all,
> I am curious if someone could shed some light onto the following:
> I am currently calculating phonons of MgO using QE 6.2.1 and NC PPs from
> the ONCV library (requirement of post-processing with YAMBO). Bulk
> properties (lattice and vibrational) are very nicely reproduced.
> When moving to slabs (1,2,3 layers with ca 20 A vacuum, dipfield
> correction applied) phonons become considerably "ugly" (very high energy
> imaginary modes). I was wondering if there is a "trick" to get better
> phonons in slabs with polar surfaces either during ph.x (I tried tighter
> convergence down to 10e-16, higher k grids in pw.x, higher q grids in ph.x
> without much success) or post-processing (not sure if the usual ASR are
> applicable in slabs)?
> Since calculations on the slabs are rather time consuming any suggestion
> is very welcome!
> A nice weekend to everyone from San Sebastian,
> Chris
> --
> Postdoctoral Researcher
> Center for Quantum Nanoscience, Institute for Basic Science
> Ewha Womans University, Seoul, South Korea
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                    Ilya Ryabinkin
                 Postdoctoral Scholar
          Physical and Environmental Sciences
           University of Toronto Scarborough
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