Set the shape of the absorbing boundaries. Here you can set the inner
and outer bounds by setting the block as follows:
%ABShape
inner | outer | "user-defined"
%
The optional 3rd column is a user-defined expression for the absorbing
boundaries. For example, $r$ creates a spherical absorbing zone for
coordinates with ${\tt inner} < r < {\tt outer}$, and $z$ creates an
absorbing plane.
Note, values outer larger than the box size may lead in these cases to
unexpected reflection behaviours.
If no expression is given, the absorbing zone follows the edges of the
box (not valid for user-defined box).