[Wien] Fermi enregy for Gerhard !!

[Gerhard H Fecher]

The Fermi - energy is by definition the energy of the highest occupied state 
in a solid (at T=0K). (see e.g. page 13 of : S.L. Altmann Band Theory of 
Solids: An Introduction....; Clarendon Press (1991))
Therefore it has to be on top of the valence bands in semi conductors (see 
page 139 ibid.).
There is by definition no way that the Fermi energy depends on the size of the 
gap.
At least if assuming that there is not a highest occupied state above the 
highest occupied state. 

Note: This is not about semi conductors with impurities (extrinsic 
semiconductivity) that may create states right in the band gap and the Fermi 
energy then lies in the "gap". The philosophical question is whether a gap 
with states inside is what a gap should be, namely something where no states 
are.
You may also talk about non Zero temperature where you occupy states in the 
conduction band. In that case you are looking to find the "Zero" for the 
Fermi-Dirac distribution, that may be the chemical potential that does not 
always coincide with the Fermi-energy but only at 0K, at least in the 
definition I know.

In the calculations you may observe that the integration of the elctron 
density may not lead to the  integer value supposed to define the Fermi 
energy if integrating up to the top of the valence band. In that case the 
programm will need some charge density from the bottom of the conduction band
to find a inter number for the number of electrons. Therfore it seems that the 
Fermi energy jumps across the gap, e.g. when changing the lattice parameter. 
In some cases it helps to increase the number of k-points and thus improve 
the precesion of the k-space integration.

Ciao
Gerhard

As you do neither give what material you are talking about, nor how you 
determined the real gap being different from the calculated one I do not like 
to decide what is the truth and what a rumour. 


Am Mittwoch, 22. März 2006 13:54 schrieb Benali Ben:
> dear Gerhard;
>  
>  The significance of the gap energy is the width between the maximun band of 
valance and the minimum of the band conduction. Cette value is underestimated 
by the LDA or the GGA implemented in the wien2k (this is not that some rumour 
but it's real knowledge). what I desired knowledge if Fermi energy depends of 
the gap energy!!
>  
>  Ciao
>  
>  benali
>  
> 		
> ---------------------------------
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