[Pw_forum] doubt in occupations

[Nicola Marzari]

Padmaja Patnaik wrote:
> Dear all
> 
> Can anybody clarify the following:
> While doing calculations for semiconductor, in the input file what will 
> be the 'occupations' which comes under the &system. Is it correct to 
> give 'smearing' ?

Dear Padmaja,

Not really - most likely it will not hurt. If the system has a gap,
you want all the valence bands fully occupied, and the conduction
bands empty. Smearing is used in metals, or occasionally to
help converge some molecular systems where the homo and lumo
are close by.

If you use smearing, and fermi dirac with a small temperature
(e.g. equiv to 300K), in silicon you'll get in any case that
all valence is practically full, with tiny fraction of electrons
excited across the gap. The calculation is more expensive, because
you calculate more states than needed (i.e. some at the bottom
of the conduction band).

Please add your affiliation to emails to this list.

			nicola

-- 
---------------------------------------------------------------------
Prof Nicola Marzari   Department of Materials Science and Engineering
13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu