[Pw_forum] is there a way to find the edge of conduction band from fermi level?

Giuseppe Mattioli giuseppe.mattioli at mlib.ism.cnr.it
Fri May 27 12:04:22 CEST 2011

Dear M Alaii

Please provide always your affiliation when posting.

> I was wondering whether there is a way to guess what the edge of conduction
> band is by the Fermi level that the software provides.

It depends on the system. In the case of an insulator or semiconductor you 
should draw a reasonable band structure and compare with it the Ef value 
given by the code. You can always try to fill the bands by hand with the 
correct amount of electrons...

In the case of insulating isolated systems (i.e., molecules) the LUMO orbital 
is just the first above the given Ef at gamma.

In the case of metals or semimetals you should find that the Ef value crosses 
(one of) the bands.

Of course you should also calculate a given number of unoccupied eigenstates, 
either by using a nbnd value greater than nelec/2, or by using some smearing 

Remember that enegy gaps (or HOMO-LUMO gaps) are affected by a static 
correlation error when using LDA or GGA approximations, so they often compare 
badly with optical gaps.



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