  Model calculations that include the effects of irreversible, environmental
couplings on top of a coupled-channels dynamical description of the collision
of two complex nuclei are presented. The Liouville-von Neumann equation for the
time-evolution of the density matrix of a dissipative system is solved
numerically providing a consistent transition from coherent to decoherent (and
dissipative) dynamics during the collision. Quantum decoherence and dissipation
are clearly manifested in the model calculations. Energy dissipation, due to
the irreversible decay of giant-dipole vibrational states of the colliding
nuclei, is shown to result in a hindrance of quantum tunneling and fusion.
