[WIEN]Why Fermi Energy is positive

[Peter Blaha]

Your guesses are perfectly ok.

Remember, a "bandstructure" approach deals with an infinite solid. Even at 
"infinity" there are atoms,.... 
Thus one cannot use the approach of molecular codes, which put the zero of 
energy at infinity.

Our energy zero is set quite arbitrarely such, that the average coulomb 
potential in the interstitial region is zero. For this reason we have 
positive EF, and the larger the interstital is (eg. for surface-slabs with 
large vaccum) the "more negative" EF can go.

Anyway, the absolute energies of EF or eigenvalues have no meaning!

> In fact,I Do NOT suspect that there were something wrong with WIEN2K that 
> leads to the Fermi-Energy be positive and What I wonder and much more care 
> is: Where is the ZERO point of Potential in the Crystial-Calculation and What 
> is its Physical means?
> 
> I believe just for the choice of the point of reference-potentical,part of 
> the EIGENVALUES become positive.Maybe this is a elementary question in LAPW.
> 
> Best wishes!
>                                                Sincerely Jun
> > Energies of free electrons are always positive as the kinetic energy 1/2 m 
> v^2 cannot be negytive (if neglecting inelastic approximations).
> > 
> 
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                                      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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