[Wien] External electric field (bias)

[Lukasz Plucinski]

Dear Prof. Blaha, Prof. Marks,

Thank you for your quick comments.

I am trying to do the 0001 surface of MnTe (NiAs lattice).

Making partial coverages for relaxation is hard, I don't want to deal with
folded bands. I did not do any relaxation so far, but I am not sure
relaxation with 1x1 cell it will solve the problem.

Without DFT+U there is still bulk band gap, and bulk bands change quite a
lot near VBM, so this not a solution.

I am also thinking of trying H atom on both sides of the slab.

Best,
Lukasz

On Sat, Oct 11, 2025 at 8:54 AM Peter Blaha <peter.blaha at tuwien.ac.at>
wrote:

> You post 2 questions:
>
> Your calculations of the slab without field do not give expected results ??
>
> Since we do not know any details - hard to comment. How many layers ?
> How much vacuum ?  Note: Polar surfaces are always a problem and your
> model has probably 2 inequivalent surfaces, a lot of dangling bonds, ...
>
> Polar surfaces show usually huge reconstructions / become metallic /
> have in reality stoichiometry changes or H-termination to reach
> neutrality or bond-saturation / ...
>
> A periodic model for them is always a bit problematic due to long range
> coulomb forces and dipole effects ....
>
> You should be sure your model is realistic and applicable before using
> an E-field.
> ------------------------------------------
> What should I use for efield,.... ?
>
> Start with the values in the UG. Note: Efield is in Ry units. You have
> to consider your unit cell length (actually half of it), and the real
> field is then   Efield/c/2. Without screening, this will introduce a
> force of    efield/c/2 on the atoms and thus I'd suggest to use an
> efield to get 50-100 mRy/bohr.
> You will see how much of this is screened by the electron density
> redistribution during scf and eventually you can relax the structure to
> see the change of positions due to the field. For the latter, I'd fix
> the middle of the slab, otherwise the whole slab may translate in the
> field ....
>
> You probably don't have inversion and mirror symmetry, but in case you
> do, break it manually !!!
>
> Am 10.10.2025 um 20:05 schrieb Lukasz Plucinski:
> > Dear All,
> >
> > I have been trying to calculate a slab of a polar semiconductor.
> > Unfortunately, there seems to be an effect of an electric field across
> > the slab.
> >
> > I see that one can add external E field by including line 4 in case.in0
> > file: "IFIELD, EFIELD, WFIELD". I understand that my slab should be
> > positioned at around 1/4 of the unit cell (so, a lot of vacuum), to
> > make use of the zig-zag potential.
> >
> > So far I calculated the slab (without E field), and I can see a ladder
> > of bands, instead of something resembling the "projected bulk band
> > structure". Is there a way to read some parameters from this slab, in
> > order to have an educated guess for the "IFIELD, EFIELD, WFIELD" values?
> >
> > Best,
> > Lukasz
> >
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> --
> -----------------------------------------------------------------------
> Peter Blaha,  Inst. f. Materials Chemistry, TU Vienna, A-1060 Vienna
> Email: peter.blaha at tuwien.ac.at
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