[Wien] External electric field (bias)
Laurence Marks
laurence.marks at gmail.com
Sat Oct 11 12:53:15 CEST 2025
GIGO
The bulk terminations are highly unstable, so your results will be
meaningless. You need to test:
1. 2x1 terminations with 1/2 occupancy at the surface.
2. c2x2 also with 1/2 occupancy
3. 2x2 octahedral similar to MgO (111)
You will need to relax positions and calculate free energies. This is not
hard, but you cannot cut corners and expect anything useful, my hunch is
that 3 will be most stable.
___
Emeritus Professor Laurence Marks (Laurie)
Department of Materials Science and Engineering, Northwestern University
www.numis.northwestern.edu
"Research is to see what everybody else has seen, and to think what nobody
else has thought" Albert Szent-Györgyi
On Sat, Oct 11, 2025, 03:16 Lukasz Plucinski <lukasz.galaxy.s3 at gmail.com>
wrote:
> 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
>> WWW: http://www.imc.tuwien.ac.at WIEN2k: http://www.wien2k.at
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>>
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