[Pw_forum] how to saturate dangling bonds in surface

[Malgorzata Wierzbowska]

On Wed, 30 Aug 2006, stefano fabris wrote:


  Since pwscf is a periodic program, Eduardo has obviously a problem
with z-direction. Broken Mg-O bonds are not saturated.
For the hydrogen saturation method one may put O-H bond about 0.95Ang.
Franc Rittner used that method for rutile to saturate clusters
in the embeded cluster method, but for Eduardo it applies to the surface.
The method should work more/less for the adsorption of neutral
molecules on the slab.

For the adsorption of charged species, however, the H saturation method
is far inaccurate.
In the old Evjen paper there is a proof that the calcelation
of moments up to the quadrupole is necessary for the convergence of
the potential above the surface. Dipol moment is not enough!
HM Evjen Phys Rev vol. 39, p. 675 (1932).
I would recommend to read this paper first, because many good scientists
recently forgot it and publish wrong results.

For the methods of cancellation of electrostatic moments one can have a
look to paper by Claudine Noguera and to her book, and to
L. Stolarczyk and L. Piela Int. J. of Quant. Chem. vol. 22 p. 911 (1982).

   Regards,
   Gosia

PS. Eduardo, if you like I may send you my thesis about these things.


>
>> 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,
> 			Stefano
>
>
>> 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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