  Stellar-remnant black holes (BH) in dense stellar clusters comprise a natural
setup to trigger general-relativistic (GR) inspiral and merger of binary black
holes (BBH), detectable by the LISA and the LIGO-Virgo, through dynamical
encounters inside such environments. In this work, the intricacies of such
dynamical interactions are probed utilizing realistic, self-consistent,
post-Newtonian, direct N-body evolutionary models of young massive and open
stellar clusters. Particularly, the configurations of the compact subsystems,
that drive the in-cluster GR BBH coalescences, are tracked on the fly. Such an
approach reveals that the GR coalescences within the open clusters take place
primarily via chaotic interactions involving triple BH systems. Although less
frequently, such mergers are found to happen also in higher-order subsystems
such as quadruples and in subsystems involving non-BH members; the mergers can
themselves be BH---non-BH, which events would leave electromagnetic signatures.
Close, fly-by encounters inside the clusters can also make BBHs and other types
of double-compact binaries temporarily post-Newtonian; such binaries would
potentially contribute to the GW background for the LISA and the PTA. These
calculations, furthermore, suggest that open clusters are potential hosts for
not only detached BH---main-sequence binaries, as recently identified in the
globular cluster NGC 3201, but also a wide variety of other types of
remnant---non-remnant binaries, which are assembled via dynamical interactions
inside the clusters and which have the prospects of being discovered in
radial-velocity surveys.
