If NSIM is specified in VASP.4.4 and newer versions,
the RMM-DIIS algorithm (IALGO=48) works in a blocked mode.
In this case, NSIM bands are optimized at the same time. This allows to
use matrix-matrix operations instead of matrix-vector operation
for the evaluations of the non local projection operators in real space,
and might speed up calculations on some machines. There should be no difference
in the total energy and the convergence behavior between NSIM=1 and NSIM
1, only
the performance should improve.