[Pw_forum] isolated atom

[Nicola Marzari]

ali kazempour wrote:
>
>  Hi
> I want to get binding energy of  O2 mulecule .For this I need the 
> correct value for isolated atom energy. But when I change(reduce) the 
> smearing ,the value of  energy increase. So I DONT know which smearing 
> should I use? the larger or the smaller?
> thanks


Smearing is used primarily to smooth the fermi discontinuity in metals, 
allowing
to use coarser meshes of k-points. From this point of view, for an atom 
you'll
have gamma sampling and no k-points issues.

A secondary effect of smearing is that it helps reach selfconsistency - 
especially
in atoms or molecules where the HOMO and LUMO are close and could swap
order during selfconsistency.

What you want is to use a smearing that is quite a bit smaller than the 
converged
HOMO-LUMO gap - when that happens, the HOMO is full, the LUMO is
empty, and the energy E or the free energy E-TS do not change as you vary
the fictitious smearing temperature T.

If you have trouble reaching self-consistency in this case, use a larger 
smearing,
and then restart from your converged results with the smaller one.

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