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##

Number of bands NBANDS

One should chose NBANDS so 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 to set 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 sensible to the
number of bands than the default CG algorithm (IALGO=8).

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