[Pw_forum] band energies

[Nicola Marzari]

Dear Helen,

there are several ways to look at this - most thorough discussion would
be Caspar Fall PhD thesis at EPFL (www.epfl.ch) - it's available on the
web. Also, for a recent review, citing most of the relevant literature, 
you could look at http://arxiv.org/abs/0801.1077 . The papers
by Baldereschi/Baroni/Resta/Peressi from the late '80s onwards on
homo and heterosemiconductor junctions contain the original discussion
of these issues.

In short - you can refer the homo of your bulk semiconductor to another
property that is easy to transfer across non-bulk environment - often
the average value of the Hartree or of the Kohn-Sham potentials. Suppose
the HOMO were 3.5 eV above the Hartree bulk average.

Then, in a slab calculation, you can find out what is the average 
Hartree well inside the slab (where it is bulk-like), and well outside
the slab, what it reaches the vacuum level. Suppose this difference
is 5eV. Then, the HOMO is 1.5eV below the vacuum level - i.e. it takes
1.5eV to bring the top valence electron well outside the solid (across 
the surface used in your slab calculation - these quantities are surface
dependent).

				nicola



Helen wrote:
> Hello,
> So far I've been calculating bandgaps and so have been using the 
> differences in band-energies, now I want to compare the absolute value 
> of the HOMO in semiconductors with experimental workfunctions. How can I 
> know what the absolute zero value of the bandenergies is? Is there any 
> meaning to the absolute values given in the output file or are only the 
> band energy differences relevant?
> Thanks for your help,
> Dr. Helen Eisenberg,
> Fritz Haber Center, Hebrew University. 
> 
> 
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Prof Nicola Marzari   Department of Materials Science and Engineering
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