# [QE-users] Dipole corrections (dipfield) and the position of the slab

Thomas Brumme thomas.brumme at uni-leipzig.de
Fri Apr 27 10:59:41 CEST 2018

```Dear Chris,

both planes of the dipole (the one at emaxpos and the one with the
opposite charge at emaxpos+eopreg) have to be in the vacuum region.
In fact, there should be enough space such that the wavefunctions are
essentially zero at the dipole planes. However, if the dipole is too large,
charge can spill into the vacuum region as plane waves are not localized
on the system and the charge could be in a lower energy state at the
dipole. In other words. don't use 50 Angstrom of vacuum as this will lead
to a very low minimum in the total potential at the dipole.

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.85.045121

In this paper charged systems are discussed but similar things apply to
the dipole correction. Thus, if your system is centered at 50% of the
cell, center the dipole at zero and converge things with increasing the
size along z.

Regards

Thomas

On 26.04.2018 14:00, Christoph Wolf wrote:
> Dear all,
>
> After trying for a few days I am still a bit puzzled by the "proper
> application" of the dipole correction. To test this I have made a
> sheet of graphene added hydrogen below and fluorine above. I then
> apply the following corrections:
>
>     tefield = .true.
>     dipfield =.true.
>
> and
>
>   eamp        = 0.00
>   edir        = 3
>   emaxpos     = 0.80 !(=16 Angstrom)
>   eopreg      = 0.10 ! (=2 Angstrom)
>
> The cell is 20 A in total. As I shift the layer from 0% of the cell to
> 50% cell (whilst keeping above emaxpos at 80% and eopreg at 10% of the
> cell) the Fermi level shifts slightly (~0.2-0.5 eV difference) and the
> electrostatic potential (pp.x plot num 11 and then planar average
> using average.x as in the work-function example) is only "flat" in the
> vacuum region when the sample is about 3A from the bottom of the cell
> (i.e. the z coordinate of atoms has to be larger than 3 A).
>
> Reading the pw.x input I was under the impression that only emaxpos
> has to fall into the vacuum but is there also a "rule of thumb" for
> eopreg?
>
>
> Best,
> Chris
>
>
> PS: I saw the related discussion, but it does not really answer this I
> think... http://qe-forge.org/pipermail/pw_forum/2009-December/089951.html
> --
> Postdoctoral Researcher
> Center for Quantum Nanoscience, Institute for Basic Science
> Ewha Womans University, Seoul, South Korea
>
>
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--
Dr. rer. nat. Thomas Brumme
Wilhelm-Ostwald-Institute for Physical and Theoretical Chemistry
Leipzig University
Phillipp-Rosenthal-Strasse 31
04103 Leipzig

Tel:  +49 (0)341 97 36456

email: thomas.brumme at uni-leipzig.de

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