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

[Thomas Brumme]

Dear Chris,

in the end it (of course) doesn't (and shouldn't) matter where you 
center what.
You can also center the system at z=0 and the dipole at z=0.5. Just take 
care
that the distance between system and dipole is large enough.

Regards

Thomas

On 27.04.2018 12:06, Christoph Wolf wrote:
> Dear Thomas,
>
> thank you for your detailed reply!
>
> If I understood this correctly, the ideal situation would be to have 
> the slab in the center of the cell
>
> Atom xx yy 0.5
>
> (in crystal coordinates)
>
> to center the dipole at 0% (=100% due to PBC) a reasonable choice 
> would be emaxpos=0.95 and eopreg=0.10. In the dipole example they 
> located the atoms around z=0 of the cells and put the dipole close to 
> the center of the cell.
>
> Muchas Gracias/Vielen Dank from Spain,
>
> Christoph
>
>
>
>
> On Fri, Apr 27, 2018 at 10:59 AM, Thomas Brumme 
> <thomas.brumme at uni-leipzig.de <mailto:thomas.brumme at uni-leipzig.de>> 
> wrote:
>
>     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.
>     See also this paper:
>
>     https://journals.aps.org/prb/abstract/10.1103/PhysRevB.85.045121
>     <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?
>>
>>     Thanks in advance for your help!
>>
>>     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
>>     <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 <mailto:thomas.brumme at uni-leipzig.de>
>
>
>
>
> -- 
> Postdoctoral Researcher
> Center for Quantum Nanoscience, Institute for Basic Science
> Ewha Womans University, Seoul, South Korea

-- 
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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