[QE-users] smearing vs. tetrahedron - weird behavior

[Christoph Wolf]

Dear Nicola,

for smearing I used the default Gaussian one; The outliers seem to come
from the tetrahedron method for a few "special" k-grids. I have to say I
never observed this for any metal slabs I worked on so this seems to be a
very particular case: a few isolated impurity bands (basically
dispersion-free) inside an insulator band-gap.

Thanks for the information regarding mv and mp smearing that is certainly
helpful!

Best,
Chris

On Mon, Mar 25, 2019 at 1:46 AM Nicola Marzari <nicola.marzari at epfl.ch>
wrote:

>
>
> Is 8.2 (the outliar) given by the tetrahedron method?
>
> For insulators with a large band gap, if one uses mp or mv smearing,
> there is the possibility that the code chooses a fermi energy just
> below the top of the valence (with mv or mp) and just above the bottom
> of the conduction (with mp), rather than in the middle of the gap
> (albeit rare, this can happen, due to the fact that mv in these cases
> can have two solutions for the fermi energy, and mp can have three).
>
> But your case seem to show a problem with the tetrahedron method, and
> not sure that what is the problem.
>
>                         nicola
>
>
> On 24/03/2019 04:21, Christoph Wolf wrote:
> > Dear all,
> >
> > I am facing something rather interesting. My system is an insulator
> > supercell with a metallic impurity. I tried to converge the Fermi energy
> > wrt. to k-points and smearing in particular for "low temperatures" (~20
> > K). I observed the following behavior for a chosen number of k-points
> > (e.g. 6 6 6 0 0 0): with smaller smearing the fermi level approaches the
> > same value as the fermi level using the tetrahedron method (i.e. the
> > tet_opt method represents 0 Ry smearing).
> >
> > however I found that for higher values for the k-point grid this does
> > not seem to always work. For example for a 25 grid the value for the
> > fermi level differs by ~0.8 eV! Is there a reason for this behavior?
> >
> > #k-grid kpnts Ef: smear tet
> > 8      35  9.004  8.994
> > *25  455  8.974  8.201*
> > 26  560  9.0068.996
> >
> > Your help is very much appreciated!
> >
> > Best wishes,
> > Chris
> > --
> > Postdoctoral Researcher
> > Center for Quantum Nanoscience, Institute for Basic Science
> > Ewha Womans University, Seoul, South Korea
> >
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> >
>
>
> --
> ----------------------------------------------------------------------
> Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
> Director, National Centre for Competence in Research NCCR MARVEL, EPFL
> http://theossrv1.epfl.ch/Main/Contact http://nccr-marvel.ch/en/project
>


-- 
Postdoctoral Researcher
Center for Quantum Nanoscience, Institute for Basic Science
Ewha Womans University, Seoul, South Korea
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