[Wien] External electric field (bias)
Lukasz Plucinski
lukasz.galaxy.s3 at gmail.com
Sat Oct 11 10:15:40 CEST 2025
Dear Prof. Blaha,
I did not look at the forces yet, only started with the bulk truncated
bulk. I have been interested in a qualitative result, proper relaxation is
time consuming and I have little experience.
The problem is that in slab calculation I see a ladder of bands and a
decreased band gap. But what one should see (or what I would prefer to see)
is something that resembles projected bulk bands, plus some surface states.
I calculated projected bulk bands, and they don't match well with the slab
bands. I test grep :MM case.scf to make sure there is AFM phase.
At the moment I do "Mn-Te-....-Mn-Te" slab, so both terminations in
one slab. Having Te termination on both sides led to similar problem with
bands. I did not test Mn termination on both sides yet.
Best,
Lukasz
On Sat, Oct 11, 2025 at 10:00 AM Peter Blaha <peter.blaha at tuwien.ac.at>
wrote:
> 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 <mailto:peter.blaha at tuwien.ac.at>
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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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