[QE-users] PbTiO3 slab in vacuum with assume_isolated='2D'

[Lucian D. Filip]

Hello,

I am looking at a PbTiO3 slab in vacuum. The slab is in a polarized 
state and I am trying to obtain the Hartree potential with the dipole 
correction applied. I have seen that the assumed_isolated = '2D' option 
should do just that. However, I do not understand the result I am 
getting. In the attached picture I have plotted the Hartree potential 
averaged along the z axis, for the polarized state and the 
centro-simmetric state with polarization zero.

The slab is 5 PTO unit cells thick and there are 20 Angstroms of vacuum 
on each side. The vacuum region is well above the requirements of 
assume_isolated ='2D'. Can anyone explain two things:

1) why are there two regions in vacuum looking like a staircase.

2) why is there an electric field still present?


The weird artefacts at the edges of the supercell are due to the 
averaging procedure (i am still working on it, not sure where it is 
coming from).


Best regards,

Lucian


-- 
Dr. Lucian Dragos Filip
National Institute of Materials Physics
Atomistilor str. 405A, PO Box MG. 7
Magurele, 077125
Bucharest, Romania
E-mail:lucian.filip at infim.ro
Website:https://lucianfilip.wordpress.com/
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