Roberto,
please, show your fdf file and specify on how many processors you are
running.
Usually there is no slowness related to optical calculation.
However, I believe it takes a bit more memory, so if you are running on
a desktop pc the HDD virtual memory may be used for memory, thus slowing
everything down.
Another thing is how many bands you use for optical calculations.
Usually, you need the lowest 10% of all the bands (i.e. only several
bands beyond the bandgap), but if you don't specify this explicitly
SIESTA will be computing transition elements for all the pairs for
VB-to-anything beyond bandgap.
Alex.
