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

[Christoph Wolf]

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  9.006 8.996

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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