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7.33 ISMEAR, SIGMA tag


SIGMA = 0.2

ISMEAR determines how the partial occupancies tex2html_wrap_inline4645 are set for each wavefunction. For the finite temperature LDA SIGMA determines the width of the smearing in eV.


Gaussian smearing
method of Methfessel-Paxton order N Mind: For the Methfessel-Paxton scheme the partial occupancies can be negative.

partial occupancies are read in from INCAR, and kept fixed throughout run.

There should be a tag


and for spin-polarized calculations


in the INCAR file supplying the partial occupancies for all bands and k-points. The band-index runs fastest. The partial occupancies must be between 0 and 1 (for spin-polarized and non-spin-polarized calculations). Mind: Partial occupancies are also written to the OUTCAR file, but in this case they are multiplied by 2, i.e. they are between 0 and 2.

make a loop over the different smearing-parameters supplied in the INCAR file.

There should be a tag


in the INCAR file, supplying different smearing parameters. IBRION is set to -1 and NSW to the number of supplied values. The first loop is done using tetrahedron method with Blöchl corrections.

tetrahedron method without Blöchl corrections
tetrahedron method with Blöchl corrections

For the calculation of the total energy in bulk materials we recommend the tetrahedron method with Blöchl corrections (ISMEAR=-5). This method also gives a smooth nice electronic density of states (DOS). The only drawback is that the methods is not variational with respect to the partial occupancies, therefore the calculated forces might be wrong by a few percent for metals. For the calculation of phonon frequencies based on forces we recommend the method of Methfessel-Paxton (ISMEAR>0). For semiconductors and insulators the forces are always correct, because partial occupancies do not vary!

The method of Methfessel-Paxton (MP) also results in a very accurate description of the total energy, nevertheless the width of the smearing (SIGMA) must be chosen carefully (see also 9.4). Too large smearing-parameters might result in a wrong total energy, small smearing parameters require a large k-point mesh. SIGMA should be as large as possible, but the difference between the free energy and the total energy (i.e. the term ' entropy T*S') in the OUTCAR should be negligible (usually less than 1 meV/atom). In most cases N=1 and N=2 leads to very similar results. The method of MP is also the method of choice for large super cells. In this case the tetrahedron method is not applicable if less than three k-points are used.

Mind: Avoid to use ISMEAR>0 for semiconductors and insulators, it might result in problems, because this function gives occupancies which are larger than 2. For insulators you can always use the tetrahedron method (ISMEAR=-5).

The Gaussian smearing (GS) method leads in most cases also to reasonable results. Within this method it is necessary to extrapolate from finite SIGMA results to SIGMA=0 results. There is an extra-line in the OUTCAR file 'energy(SIGMA tex2html_wrap_inline4651 )' stating the results of this extrapolation. Large SIGMA values lead to a similar error as the MP scheme, but in contrast to the MP scheme there is no way to figure out, how large the error due to the smearing is. Therefore the method of MP seems to be superior to the GS method. In addition forces and stress are consistent with the free energy and not the energy for SIGMA tex2html_wrap_inline4351 0. Usually the Methfessel-Paxton is therefore easier to use.

For further considerations on the choice for the smearing method see sections 9.4,10.6. To summarize, use the following guidelines:

next up previous contents
Next: 7.34 LREAL-tag (and ROPT-tag) Up: 7 The INCAR File Previous: 7.32 EMINEMAX tag

Mon Mar 29 10:38:29 MEST 1999