[Pw_forum] Smearing for metal/insulator supercell

Nicola Marzari marzari at MIT.EDU
Thu Jan 17 14:32:47 CET 2008

> 1) How to define whether a system is (or expected to be) insulating or
> conducting by parameters of PWSCF?

You could start by using fractional occupations, and smearing, and
see, after selfconsistency, if there is a gap or not.

> 2) for metal/insulator supercell, how to set the related parameters? If
> it's smearing, should I use a as small as possible smearing?

Generally speaking, you want the result in the limit of zero smearing.
In that limit, a metallic system will usually requires an extremely fine
k-point sampling, since you are trying to integrate a discontinuous
function. So, you need to find a smearing that is not
too large to give you incorrect results, but large enough that you
can use a reasonable number of k-points to achieve full sampling
accuracy. See Chap 4 of http://quasiamore.mit.edu/phd for an introduction.

Generally speaking, a smearing of 0.3eV=0.022Ry is a conservative
starting points. PWscf uses Ry for smearing, CP (I believe) Ha.


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

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