[Pw_forum] how to saturate dangling bonds in surface

stefano fabris fabris at democritos.it
Wed Aug 30 11:07:42 CEST 2006

> Hello,
> I have a problem of surface defect in MgO. This is ionic with rocksalt
> structure. In a supercell calculation one has a real surface and an
> artificial surface that should mimic the bulk material. ?Are there
> dangling bonds in the bulk-like surface? Must they be saturated with a
> pseudo hidrogen (with charge one, at wich distance) ?
> Thanks

Dear Eduardo,

I am not sure to completely understand your problem, but here are  
some thoughts, possibly relevant. The method used to model ionic  
surfaces (and their defects) with periodic approaches depends on the  
specific surface you are interested in, specifically if it is polar  
or non polar.

The case of non polar surfaces, as the MgO (001) or the (110), is  
simpler and saturation is not really necessary. The obvious method  
used in this case is to build a slab model of the surface  
sufficiently thick so that the atoms in the middle of the slab are  
effectively bulk-like. You can check this condition in various ways,  
for example by comparing the PDOS of the central atoms in the slab  
with the PDOS calculated in a bulk supercell. In this case, you can  
often get away with a reasonable small number of layers, particularly  
with simple oxides like MgO.

The case of polar surfaces, as the MgO (111) is more delicate, since  
you have to pay attention to artificial dipole moments that, if not  
properly handled, increase monotonically with the slab thickness and  
lead to divergency of the surface energy. Also, they influence  
surface relaxations and structural relaxations around defects.  
Saturation of the dangling bonds is one possibility to cope with this  
systems, but other methods have been proposed, as for example the use  
of slabs in which the dipole moment is canceled by symmetry or by an  
external field. Here is some relevant bibliography: E. Kaxiras et  
al., PRB 35, 9625 (1987); B. Meyer and D. Vanderbilt, PRB 63, 205426  
(2001); L. Bengtsson, PRB 59, 12301 (1999).

Hope it helps. All the best,

> Eduardo A. Menendez Proupin
> Department of Physics
> Faculty of Science
> University of Chile
> Las Palmeras 3425
> Ñuñoa, Santiago
> Chile
> Phone: 56+2+978 74 11
> http://fisica.ciencias.uchile.cl/~emenendez/
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