[Pw_forum] sawlike potential in supercell calculation

[Stefano de Gironcoli]

Dear Mohamed,
  
   the saw-like potential mimic an uniform external electric field ... 
that is a field that is generated by a planar capacitor... actually an 
infinite array of planar capacitors ...
   to this end you need to specify where you want to put the capacitor 
plates (typically in the vacuum region) and what is the potential drop
between them.
   emaxpos says WHERE to put the negative capacitor plate in unit of the 
relevant unit cell.
   eopreg  says WHERE to to the positive capacitor plate (relative to 
the negative one).
   eamp is related to the electric field amplitude i.e. the potential 
drop across the capacitor.
  
stefano

Mohamed.Majdoub at mail.uh.edu wrote:
> Dear users,
>
> I really find difficulties in understanding and ambiguities on how the 
> saw like potential is implemented in pwscf. I need to apply an 
> external electric filed to a slab structure in supercell (including 
> vacuum region) in the z direction (edir =3) of amplitude 
> eamp.According to the manual, besides setting tefiled and dipfield 
> (dipole correction for slab) to be true, we need to specify the 
> following variables in &system:
> ------------------------------------------------------------------------
> edir           INTEGER
>
>                The direction of the electric field or dipole 
> correction is
>                parallel to the bg(:,edir) reciprocal lattice vector, 
> so the
>                potential is constant in planes defined by FFT grid points;
>                edir = 1, 2 or 3. Used only if tefield is .TRUE.
>
> emaxpos        REAL ( default = 0.5D0 )
>
>                Position of the maximum of the sawlike potential along 
> crystal
>                axis "edir", within the  unit cell (see below), 0 < 
> emaxpos < 1
>                Used only if tefield is .TRUE.
>
> eopreg         REAL( default = 0.1D0 )
>
>                Zone in the unit cell where the sawlike potential 
> decreases.
>                ( see below, 0 < eopreg < 1 ). Used only if tefield is 
> .TRUE.
>
> eamp           REAL ( default = 0.001 a.u. ) 0.000001 a.u
>
>                Amplitude of the electric field (in a.u. = 51.44 10^10 
> V/m )
>                The sawlike potential increases with slope "eamp" in the
>                region from (emaxpos+eopreg-1) to (emaxpos), then decreases
>                to 0 until (emaxpos+eopreg), in units of the crystal
>                vector "edir". Used only if tefield is .TRUE.
> ---------------------------------------------------------------------
> Since it is external electric field it shoud be only applied to the 
> vacuum region in the supercell as it was pointed out before 
> (http://www.democritos.it/pipermail/pw_forum/2004-July/001169.html). 
> However, it is not clear if the 0< x<1 interval is the vacuum interval 
> or the full length of the supercell (structure + vacuum). How the 
> decrease in the sawlike potential is defined? Is it in the slab or the 
> vaccum?
> Can any one familiar with this kind of calculation explain how it is 
> implemented?
>
> Thank you in advance!
>
> Mohamed Sabri Majdoub
> PhD mechanical engineering
> University of Houston
> Houston, Tx, USA
> ------------------------------------------------------------------------
>
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