[Wien] Energy level

[Peter Blaha]

> Why I ask: in photoemission experiment one can mount 2 samples on the
> same sample holder such that they are in electrical contact. If one of the
> samples is metallic and other semiconducting one can measure that the
> Fermi edge of this system lies within the optical gap of the
> semiconductor. Can WIEN calculate this position? I suppose not, but would
> like to be sure. I suppose WIEN is just putting the Fermi-level on last
> occupied band such that it is always at the VBM for semiconductors.

You may be able to calculate this by two slab calculations (one for the
metal, one for the insulator). With these calculations you can define
a common energy scale (potential in the vacuum, which is given in the
scf file) and then you ave the metallic fermi-energy as well as the
valenceband maximum on an absolute scale.

> I also noticed that the width of the bulk valence band is smaller in slab
> calculation (15 layers slab separated with 15 A of vacuum) than in bulk
> calculation by around 0.5 eV (although shape of projection is like in
> bulk calculation). Can I consider this as correct behaviour related to
> the fact that slab is actually only approximating real system?

Sounds strange!
The only explanation I would have (except that you have made some mistake)
is: By choosing a particular slab with a certain number of layers you
"define" the k-mesh in the kz direction. Suppose you use 3 layers, this
means that you have Gamma and kz=1/3 and 2/3 (of your bulk cell) in your
Gamma-point of the slab.
If it happens that the VBMax/min is e.g. at kz=0.5, then this point is NOT in
your k-mesh and the corresponding width is smaller.
Maybe an even number of layers helps in this case, but this needs further
analysis.

PS: I've changed your subscription.

                                      P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-15671             FAX: +43-1-58801-15698
Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/
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