[DFTB-Plus-User] Fermi energy for surface

[Bálint Aradi]

Hi,

> I have following issue. My system is slab of titanium dioxide surface and I
> have found that by increasing the supercell size in nonpereodic direction
> (increasing the thickness of vacuum layer between two repeated slabs) the
> obtained Fermi energy also increases. In slab approximation the vacuum layer
> should be thick enough but I really confused in choice of it. The slab
> thickness is ~ 16 Å and for two different supercell heights of ~50 Å and
> ~100 Å the corresponding Fermi energies are 3.2 eV and 3.6 eV respectively.
> Of cause for supercell height >100 Å the convergence in Fermi energy is
> better but in literature I didn’t found so thick vacuum layers. Such
> difference is not so pronounced for total energy (~2E-5 eV).

  Does the Fermi energy also change with respect to the bands of your
slab? The absolute values do not necesserily mean much. Especially in
periodic systems, due to the Ewald summation, you could have a rigid
shift of all levels when you change your supercell vectors.

  Best regards,

  Bálint

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


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