[Wien] External electric field (bias)

[Peter Blaha]

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
WWW:   http://www.imc.tuwien.ac.at      WIEN2k: http://www.wien2k.at
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