[Pw_forum] Improve convergence, slab and electric field.

[Tone Kokalj]

On Fri, 2010-08-27 at 10:29 -0500, joaquin peralta wrote:
> Dear pw_forum
> 
> I try to simulate a metallic slab under a electric field. In order to
> improve the convergence, I checked this web page :


>    edir                  = 3
>    emaxpos               = 0.9
>    eopreg                = 0.2
>    eamp                  = 0.019447
> /

This input explains why you face SCF problems: they are inevitable with
your dipole-layer specification. You have placed your dipole layer
from 0.9 to 0.9+0.2=1.1. Note that position of emaxpos/eopreg is in
internal units, in range [0,1].

This implies that you put your dipole layer starting near the bottom of
the slab and ending in the slab.

!!! The dipole-layer (i.e. region where the sawlike potential decreases)
should be located in the middle of vacuum layer. !!!

>From you other email I assume your z-lattice vector is 140 bohr long,
and your Al slab extends from z=0 to z=82 bohr. Based on this data, a
good choice of emaxpos & eopreg would be:

emaxpos=0.77
eopreg=0.05 

An important thing when applying strong electric fields (which is your
case) is to check against the electron field emission (i.e. that a
fraction of electrons doesn't spill to a dipole layer). This can be
verified by inspecting a charge density in the vacuum region (or even
better a charge density difference between a slab under electric field
and no field).

Best regards, Tone

-- 
Anton Kokalj
J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia 
(tel: +386-1-477-3523 // fax:+386-1-477-3822)

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