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

Christoph Wolf wolf.christoph at qns.science
Fri Apr 6 15:57:47 CEST 2018

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

Since calculations on the slabs are rather time consuming any suggestion is
very welcome!

A nice weekend to everyone from San Sebastian,


Postdoctoral Researcher
Center for Quantum Nanoscience, Institute for Basic Science
Ewha Womans University, Seoul, South Korea
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