EFERMI

EFERMI = MIDGAP | LEGACY | [real]
Default: EFERMI = LEGACY 

Description: Defines how the Fermi energy is calculated in VASP. It is recommended to use EFERMI = MIDGAP.

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Fermi energy in semiconductors

For semiconducting materials, the Fermi energy is not uniquely defined in the bandgap of the material. Any value that produces the correct number of electrons is allowed. By default, VASP places the Fermi energy at a somewhat arbitrary value within the bandgap. The precise value depends on values chosen for the smearing (ISMEAR and SIGMA) and the density of states (EMIN, EMAX, and NEDOS). Typically this places the Fermi energy towards the bottom of the bandgap.

You can change this behavior by setting EFERMI = MIDGAP. VASP will then put the Fermi energy in the middle of the gap because this is the most consistent with increasing the smearing SIGMA. This algorithm to determine the Fermi energy was introduced in VASP.6.4. The value of the Fermi energy should not affect the outcome of the calculation.

Fermi energy in metals

When your system does not have a gap, the algorithm to determine the Fermi energy is not very important. Since the Fermi energy is placed exactly at the value so that underneath are enough states to accommodate all electrons, this energy is well defined and both MIDGAP and LEGACY should yield the same value. However, this also means that the Fermi energy is now a quantity that needs to be converged. Here, you need to consider the smearing (ISMEAR, SIGMA) and the density of the KPOINTS mesh. This is in particular important for band structure calculations. Typically, the Fermi energy of a band structure calculation will not be the same because the k points are not a well distributed mesh. If you are interested in the properties at the Fermi energy (e.g. for transport calculations), you should compute the Fermi energy with a very dense k-point mesh. Then, correct the band structure calculation so that the Fermi energy aligns with this converged value.

Fixed Fermi energy

Occasionally, you want to compute systems with fixed Fermi energy. To this end, set EFERMI to a numeric value. A possible use case is to set this to the converged Fermi energy in a band structure calculation. Keep in mind to set ICHARG = 11 to avoid changing the number of electrons. Sometimes you may use this to introduce doping to a system.

Related Tags and Sections

ISMEAR, SIGMA, EMIN, EMAX, NEDOS

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