Reference: Computational Material Science - June Gunn Lee
Smearing
For insulators and semiconductors, charge densities decay smoothly to zero up to the gap, and the integration at k-points is straightforward. Metals, however, display a very sharp drop of occupation (1 → 0) right at the Fermi level at 0 K. Since these factors multiply the function to be integrated, it makes the resolution very difficult with PW expansions. Several schemes are available to overcome this problem, generally replacing the delta function with a smearing function that makes the integrand smoother. In these methods, a proper balance is always required: Too large smearing might result in a wrong total energy, while too small
smearing requires a much finer k-point mesh that slows down the calculation. This is in effect the same as creating a fictitious electronic temperature in the system, and the partial occupancies so introduced will cause some entropy contributions to the system energy. However, after the calculation is done with any smearing scheme, the artificially introduced temperature is brought back to 0 K by extrapolation, and so is the entropy.
Gaussian smearing
The Gaussian smearing method creates a finite and fictitious electronic temperature (∼0.1 eV) using the Gaussian-type delta function just like heating the system up a little, and thus broadens the energy levels around the Fermi level.
Fermi smearing
The Fermi smearing method also creates a finite temperature using the Fermi–Dirac distribution function (Dirac 1926; Fermi 1926) and thus broadens the energy levels around the Fermi level: Now, eigenstates near the Fermi surface are neither full nor empty but partially filled, and singularity is removed at that point during calculation. Note that electronic thermal energy is roughly kBT, which corresponds to about 25 meV at 300
Maybe this information could be useful.
Normally, orbitals are either fully occupied or empty. Fermi smearing allows orbitals to be fractionally filled, according to a step function, that depends on the ”Fermi temperature” employed.
Fermi smearing can be useful in certain problematic convergence cases where degenerecies in the orbital energies mean that it is uncertain which state the SCF should converge on. Smearing may alleviate the problem by using an average state thereby removing the need to make a discrete choice.
Reference: G A M E S S - U K USER’S GUIDE and REFERENCE MANUAL Version 7.0 January 2006.
Reference: Computational Material Science - June Gunn Lee
Smearing
For insulators and semiconductors, charge densities decay smoothly to zero up to the gap, and the integration at k-points is straightforward. Metals, however, display a very sharp drop of occupation (1 → 0) right at the Fermi level at 0 K. Since these factors multiply the function to be integrated, it makes the resolution very difficult with PW expansions. Several schemes are available to overcome this problem, generally replacing the delta function with a smearing function that makes the integrand smoother. In these methods, a proper balance is always required: Too large smearing might result in a wrong total energy, while too small
smearing requires a much finer k-point mesh that slows down the calculation. This is in effect the same as creating a fictitious electronic temperature in the system, and the partial occupancies so introduced will cause some entropy contributions to the system energy. However, after the calculation is done with any smearing scheme, the artificially introduced temperature is brought back to 0 K by extrapolation, and so is the entropy.
Gaussian smearing
The Gaussian smearing method creates a finite and fictitious electronic temperature (∼0.1 eV) using the Gaussian-type delta function just like heating the system up a little, and thus broadens the energy levels around the Fermi level.
Fermi smearing
The Fermi smearing method also creates a finite temperature using the Fermi–Dirac distribution function (Dirac 1926; Fermi 1926) and thus broadens the energy levels around the Fermi level: Now, eigenstates near the Fermi surface are neither full nor empty but partially filled, and singularity is removed at that point during calculation. Note that electronic thermal energy is roughly kBT, which corresponds to about 25 meV at 300
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