  Time-dependent density-matrix propagation is used to demonstrate, in a
schematic model of an open quantum system, that the complex potential approach
and the Lindblad dissipative dynamics are \emph{not} equivalent. While the
former preserves coherence, it is destroyed in the Lindblad dissipative
dynamics. Quantum decoherence is the key aspect that makes the difference
between the two approaches, indicating that the complex potential model is
inadequate for a consistent description of open quantum-system dynamics. It is
suggested that quantum decoherence should always be explicitly included when
modelling low-energy nuclear collision dynamics within a truncated model space
of reaction channels.
