[DFTB-Plus-User] Wurtzite ZnO surface structure relaxtion problem

[Bálint Aradi]

Dear Shaohua,

> My interest is surface reconstructions of crystals. We have used DFTB+ in finding the surface structure of covalent crystal surfaces (e.g. silicon and diamond),  DFTB+ gives very good results compare to DFT. 
> Recently we moved from covalent crystal surface to ionic, and encountered the following problem while we try to relax polar surface configurations: 
> 
> ERROR!
> -> SCC is NOT converged, maximal SCC iterations exceeded
> 

One oxygen atom apparently leaves your surface. That is very difficult
to converge, as in one SCC cycle the single oxygen has not enough
electrons in the next one it has too many, leading to charge
oscillations. Technically often you can cure it by setting a high
electronic temperature (by setting it to 1300 K and the Broyden mixing
parameter to 0.2 I could converge your structure), but the result is the
same: your original surface is unstable, one oxygen leaves the surface.

  Best regards,

  Bálint

-- 
Dr. Bálint Aradi
Bremen Center for Computational Materials Science, University of Bremen
http://www.bccms.uni-bremen.de/cms/people/b_aradi/


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