NBANDSV: Difference between revisions

Latest revision as of 11:24, 24 June 2022

NBANDSV = [integer]

Description: NBANDSV determines how many unoccupied orbitals are included in the Casida/BSE calculations or timepropagation.

For the time-propagation algorithm increasing NBANDSV only modestly increases the compute time. For BSE and Casida type calculations, the compute time grows with the third power of the number of included occupied and unoccupied bands

$(N_{\mathrm{occ}} N_{\mathrm{virtual}} N_{\mathrm{k}})^{3}$

and the memory requirements increase quadratically

$(N_{\mathrm{occ}}N_{\mathrm{virtual}} N_{\mathrm{k}})^{2}$

Please be aware that symmetry is not exploited in the BSE code, hence memory requirements can be excessive. To allow for calculations on large systems, the BSE code distributes the BSE matrix among all available cores and uses ScaLAPACK for the diagonalization.

VASP always uses the orbitals closest to the Fermi-level, and NBANDSO ( $N_{\mathrm{occ}}$ ) and NBANDSV ( $N_{\mathrm{virtual}}$ ) determines how many occupied and unoccupied orbitals are included. The defaults are fairly "conservative" and equal the total number of electrons/2 (this usually implies that all occupied state are included). For highly accurate results, NBANDSV often needs to be increased, whereas for large systems one is often forced to reduce both values to much smaller numbers. Sometimes qualitative results for bandlike Wannier-Mott excitons can be obtained even with a single conduction and valence band.

@@ Line 1: / Line 1: @@
 {{TAGDEF|NBANDSV|[integer]}}
-Description: {{TAG|NBANDSV}} determines how many unoccupied orbitals are included in the BSE calculations.
+Description: {{TAG|NBANDSV}} determines how many unoccupied orbitals are included in the Casida/[[BSE calculations]] or [[timepropagation]].
 ----
-The compute time for BSE and Cassida type calculations growswith the third power of the number of included occupied and unoccupied bands
+For the time-propagation algorithm increasing {{TAG|NBANDSV}} only modestly increases the compute time.
+For BSE and Casida type calculations, the compute time  grows with the third power of the number of included occupied and unoccupied bands
 <math> (N_{\mathrm{occ}} N_{\mathrm{virtual}} N_{\mathrm{k}})^{3} </math>
@@ Line 13: / Line 14: @@
 <math> (N_{\mathrm{occ}}N_{\mathrm{virtual}} N_{\mathrm{k}})^{2} </math>
-Please be aware that symmetry is not exploited in the BSE code, hence memory requirements can be excessive. To allow for calculations on large systems, the BSE code distributes the BSE matrix among all available cores, and uses  scaLAPACK for the diagonalization.
+Please be aware that symmetry is not exploited in the BSE code, hence memory requirements can be excessive. To allow for calculations on large systems, the BSE code distributes the BSE matrix among all available cores and uses ScaLAPACK for the diagonalization.
-VASP always uses the orbitals closest to the Fermi-level, and {{TAG|NBANDSO}} (<math>N_{\mathrm{occ}}</math>) and {{TAG|NBANDSV}} (<math>N_{\mathrm{virtual}}</math>) determines how many occupied and unoccupied orbitals are included. The defaults are fairly "conservative" and equal the total number of electrons/2 (this usually implies that all occupied state are included). For highly accurate results, {{TAG|NBANDSV}} often needs to be increased, whereas for large systems one is often forced to reduce both values to much smaller numbers. Sometimes qualitative results for band like  Wannier-Mott excitons can be obtained even with a single conduction and valence band.
+VASP always uses the orbitals closest to the Fermi-level, and {{TAG|NBANDSO}} (<math>N_{\mathrm{occ}}</math>) and {{TAG|NBANDSV}} (<math>N_{\mathrm{virtual}}</math>) determines how many occupied and unoccupied orbitals are included. The defaults are fairly "conservative" and equal the total number of electrons/2 (this usually implies that all occupied state are included). For highly accurate results, {{TAG|NBANDSV}} often needs to be increased, whereas for large systems one is often forced to reduce both values to much smaller numbers. Sometimes qualitative results for bandlike  Wannier-Mott excitons can be obtained even with a single conduction and valence band.
-== Related Tags and Sections ==
+== Related tag and articles ==
 {{TAG|NBANDSO}},
-{{TAG|BSE calculations}}
+[[BSE calculations]],
+[[timepropagation]]
-== Example Calculations using this Tag ==
+{{sc|NBANDSV|Examples|Examples that use this tag}}
-{{TAG|Dielectric properties of Si using BSE}}, {{TAG|Model BSE calculation on Si}}
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-[[The_VASP_Manual|Contents]]
+[[Category:INCAR tag]] [[Category:Many-body perturbation theory]][[Category:Bethe-Salpeter equations]]
-[[Category:INCAR]] [[Category:BSE]]