[Pw_forum] sawlike potential in supercell calculation

[Mohamed.Majdoub at mail.uh.edu]

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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