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

[Nicola Marzari]

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