  The conditions for optimal reflection-free complex-absorbing potentials
(CAPs) are discussed. It is shown that the CAPs as derived from the
smooth-exterior-scaling transformation of the Hamiltonian,[J. Phys. B. 31, 1431
(1998)], serve as optimal reflection-free CAPs (RF-CAPs) in wave-packet
propagation calculations of open systems. The initial wave packet, can be
located in the interaction region (as in half collision experiments) where the
CAPs have vanished or in the asymptote where the absorbing potential non zero.
As we show the optimal CAPs can be introduced also in the region where the
physical potential has not vanished. The un-avoided reflections due to the use
of finite number of grid points (or basis functions) are discussed. A simple
way to reduce the "edge grid" reflection effect is described.
