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Dear Experts,

recently I came across a defect in silicon, which has two stable states, depending on the occupation of the two nearly degenerate defect levels. I tried to switch of occupation smearing by setting SIGMA = 0.0 (with ISMEAR = 0 or -1), to get only one state occupied with one electron (instead of both nearly half-and-half), but it did not seem to work. Finally I've managed with prescribed occupation (FERWE, FERDO), but is there a way to switch off smearing without that? (I'm using VASP 5.4.4.)

Best regards

Peter Deák

If you use the tetrahedron method there is no smearing, so it should be able to find the correct occupancies even for very small gaps. A very small smearing should also work, but SIGMA = 0 has a special meaning and is used for fixed occupancies. The relevant threshold is 1e-5 so anything smaller than that will not work as intended.