[Pw_forum] Convergence problem in Si slab structure

[Giovanni Cantele]

> On 13 May 2015, at 02:41, reza vatan <rezavatan64 at gmail.com> wrote:
> 
> Dear QE users,
> 
> I'm trying to optimize a Silicon slab structure with 13 atomic layers in each slab (attached file), but so far I have no success in optimizing because of convergence problem in SCF cycle. Does any one know who I can fix the problem.
> 
> Many thanks,
> Reza  
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As far as I understand you are dealing with the Si(100) surface, aren’t you?

In the case, you might want to passivate the two sides of the slab with hydrogens. If not, dangling bonds that necessarily appear can be more difficult to converge. Also, consider that for that surface, surface Si atoms are known to form dimers, that is, to pair and form surface supercells (2x1, 2x2, 4x2, see the literature).

If instead you want to converge exactly the system shown in your input file, you should better use occupations=’smearing’ (otherwise the degauss variable you next specify is useless!). Even though your surface is not metallic, that can help to converge when there are near gap electronic states, that “oscillate” in the scf convergence procedure.

Moreover, the cutoff you use might be insufficient to get converged results.

Finally, to save computing time, the 4x4x4 k-point grid can be replaced with 4x4x1, because sampling the Brillouin zone along the direction normal to the surface is useless, if the periodic replica of the slab are separated by vacuum space (z direction is in this case a “non-periodic” direction, sampled with a periodic supercell).

Giovanni


-- 

Giovanni Cantele, PhD
CNR-SPIN
c/o Dipartimento di Fisica
Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
e-mail: giovanni.cantele at spin.cnr.it
Phone: +39 081 676910
Skype contact: giocan74

ResearcherID: http://www.researcherid.com/rid/A-1951-2009
Web page: http://people.na.infn.it/~cantele