[Pw_forum] fermi energy

Agostino Migliore amigliore at cmm.upenn.edu
Fri Jan 25 20:10:44 CET 2008

Hello Rudra

With reference to the level structure of a molecule (indeed, only for a
very large molecule, with several thousands of atoms, you could speak
about a level structure which approximately resembles a "band" structure,
although it is still not rigorously the case: it is not an infinite
system), a chemical potential can be defined which lies at mid HOMO-LUMO
gap for integer occupation numbers. It lies still at mid gap for small
brodaning of fractional occupations, when only HOMO and LUMO are
fractionally occupied, although it is no more the case for sufficiently
high broadening, in every broadening scheme (as you can get from the
expression of fractional occupations in the chosen broadening scheme;
e.g., a good reference are Phys Rev B 58, 13459 and references therein).

Agostino Migliore

> when i am plotting band structure for non-metallic molecules, how should i
> choose fermi energy?
> --
> Rudra
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