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In the case of the metal oxide which has large band gap, I'm usually using ISMEAR=0 and SIGMA=0.05 eV. However, in the case of slab model of the same metal oxide, there usually exists partially occupied states near the Fermi level. In these cases, which ISMEAR should I use?

you can still use ISMEAR=0 but please be careful to choose a SIGMA which is small enough NOT to bridge the gap. In any case, you can use ISMEAR=-5 if you are sure that the system has a gap above the Fermi level.