[Pw_forum] Applying dipole correction to slab calculations

[CLAUDIA BUNGARO]

As you rightgully say the dipole of the slab changes during the
self-consistency run. You can use the option tefield=.true.  to add a
saw-like potential along the direction normal to the surface, V(z). The
saw-like potential changes during selfconsistency so to keep
the EXTERNAL electric field equal to a fixed value, given in the input
parameter eamp. To simulate an isolated slab set eamp=0 (zero external
electric field). The saw-like potential (i.e. the dipole potential) is a
function of both the dipole moment of the slab and the desired external
electric field.

Beside tefield and eamp you should set also: edir, emaxpos, and eopreg.
The edir parameter is the direction perpendicular to the slab
And emaxpos and eopreg are two parameters to define the detail of the
saw-like shape.
The position of the maximum of the sawlike potential within the
unit cell ( emaxpos) should be in the middle of the
vacuum region.
And the part of the unit cell where the sawlike potential has an inverse
slope than where the slab is (eopreg) should be a small percentage of the
vacuum region.

Usefull refs are the one cited in the routine add_efield.f90
!               (See Bengtsson PRB 59, 12 301 (1999) and
!                    Meyer and Vanderbilt, PRB 63, 205426 (2001).

A comment for the pwscf group :)
The emaxpos and eopreg parameters and their relationship to the saw-like
potential are not really clear, I think a reference to some paper or some
more hint would be helpful.

Hope this helps

__________________________________________________
Dr. Claudia Bungaro
Dept. of Physics and Astronomy, Rutgers University
136 Frelinghuysen Road, Piscataway,  NJ 08854-8019

tel: +1 732 445 4197          fax: +1 732 445 4343
email: bungaro at physics.rutgers.edu
__________________________________________________


On Thu, 8 Jul 2004, Aloysius Soon Sze Lok wrote:

> Dear all,
> I'm working on adsorption studies on metal oxides and would like to use PWSCF
> to run calculations on these surfaces.
> I have gone through the manual and the pw_docs and found a few input
> variables, like 'tefield', 'emax' and others, were referred to applying an
> efield to supercell.
> However, I cannot understand how this is used for dipole-correction in slab
> calculations. Are we to specify the experimental dipole value to correspond
> to the efield strength but this doesn't seem to be the point as the dipole
> will change as the calculation runs. And by specifying the artificial efield
> strength, will we over/under-correct the dipole in the cell?
>
> Please pardon my questions as I'm not sure how to implement this in my
> calculations. Much guidance and direction will be deeply appreciated.
>
>
> Many thanks in advance.
>
>
>
> --
> Aloysius Soon SL
> Postgraduate Student
> Structural & Computational Chemistry
> Chemistry Department
> The University Of Auckland, New Zealand
> Phone: +64 9 373 7599 ext 88291
> Email: a.soon at auckland.ac.nz
>
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