NBANDS: Difference between revisions
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One should choose {{TAG|NBANDS}} such that a considerable number of empty bands is included in the calculation. As a minimum we require one empty band. VASP will give a warning, if this is not the case. | One should choose {{TAG|NBANDS}} such that a considerable number of empty bands is included in the calculation. As a minimum we require one empty band. VASP will give a warning, if this is not the case. | ||
{{TAG|NBANDS}} is also important from a technical point of view: In iterative matrix-diagonalization schemes eigenvectors close to the top of the calculated number of vectors converge much slower than the lowest eigenvectors. This might result in a significant performance loss if not enough empty bands are included in the calculation. Therefore we recommend | {{TAG|NBANDS}} is also important from a technical point of view: In iterative matrix-diagonalization schemes eigenvectors close to the top of the calculated number of vectors converge much slower than the lowest eigenvectors. This might result in a significant performance loss if not enough empty bands are included in the calculation. Therefore we recommend setting {{TAG|NBANDS}} to {{TAG|NELECT}}/2 + NIONS/2, this is also the default setting of the makeparam utility and of VASP.4.X. This setting is safe in most cases. In some cases, it is also possible to decrease the number of additional bands to NIONS/4 for large systems without performance loss, but on the other hand transition metals do require a much larger number of empty bands (up to 2*NIONS). | ||
To check this parameter perform several calculations for a fixed potential ({{TAG|ICHARG}}=12) with an increasing number of bands (e.g. starting from {{TAG|NELECT}}/2 + NIONS/2). An accuracy of | To check this parameter perform several calculations for a fixed potential ({{TAG|ICHARG}}=12) with an increasing number of bands (e.g. starting from {{TAG|NELECT}}/2 + NIONS/2). An accuracy of should be obtained in 10-15 iterations. Mind that the RMM-DIIS scheme ({{TAG|IALGO}}=48) is more sensitive to the number of bands than the default CG algorithm ({{TAG|IALGO}}=38). | ||
{{sc|NBANDS|Examples|Examples that use this tag}} | {{sc|NBANDS|Examples|Examples that use this tag}} | ||
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[[The_VASP_Manual|Contents]] | [[The_VASP_Manual|Contents]] | ||
[[Category:INCAR]][[Category:Electronic | [[Category:INCAR]][[Category:Electronic minimization]][[Category:Dielectric properties]] | ||
Revision as of 14:31, 6 April 2022
NBANDS = [integer]
| Default: NBANDS | = NELECT/2+NIONS/2 | non-spinpolarized |
| = 0.6*NELECT+NMAG | spinpolarized |
Description: NBANDS determines the actual number of bands in the calculation.
One should choose NBANDS such that a considerable number of empty bands is included in the calculation. As a minimum we require one empty band. VASP will give a warning, if this is not the case.
NBANDS is also important from a technical point of view: In iterative matrix-diagonalization schemes eigenvectors close to the top of the calculated number of vectors converge much slower than the lowest eigenvectors. This might result in a significant performance loss if not enough empty bands are included in the calculation. Therefore we recommend setting NBANDS to NELECT/2 + NIONS/2, this is also the default setting of the makeparam utility and of VASP.4.X. This setting is safe in most cases. In some cases, it is also possible to decrease the number of additional bands to NIONS/4 for large systems without performance loss, but on the other hand transition metals do require a much larger number of empty bands (up to 2*NIONS).
To check this parameter perform several calculations for a fixed potential (ICHARG=12) with an increasing number of bands (e.g. starting from NELECT/2 + NIONS/2). An accuracy of should be obtained in 10-15 iterations. Mind that the RMM-DIIS scheme (IALGO=48) is more sensitive to the number of bands than the default CG algorithm (IALGO=38).