[Wien] External electric field (bias)

[Peter Blaha]

Mn or Te terminated ???
Same termination on both sides of the slab ?
I'd guess Mn is less problematic in principle.

How big are the forces on the surface atoms ???
This gives you an estimate about your structure.

Am 11.10.2025 um 09:42 schrieb 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 
> <mailto: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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