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###

NBANDSO and NBANDSV

The compute time for BSE and Cassida type calculations grows
with the third power of the number of included occupied and unoccupied
bands

and the memory requirements increase quadratically

Please be aware that symmetry is not exploited by 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` (
) and `NBANDSV` (
)
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 band like Wannier-Mott excitons can be
obtained even with a single conduction and valence band.

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N.B. Requests for support are to be addressed to: vasp.materialphysik@univie.ac.at