  We have carried out simulations of the coalescence between two relativistic
clusters of collisionless particles using a 3D numerical relativity code. We
have adopted a new spatial gauge condition obtained by slightly modifying the
minimum distortion gauge condition proposed by Smarr and York and resulting in
a simpler equation for the shift vector. Using this gauge condition, we have
performed several simulations of the merger between two identical clusters in
which we have varied the compaction, the type of internal motion in the
clusters, and the magnitude of the orbital velocity. As a result of the
coalescence, either a new rotating cluster or a black hole is formed. In the
case in which a black hole is not formed, simulations could be carried out for
a time much longer than the dynamical time scale, and the resulting
gravitational waveforms were calculated fairly accurately: In these cases, the
amplitude of gravitational waves emitted can be $\sim
10^{-18}(M/10^6M_{\odot})$ at a distance 4000Mpc, and $\sim 0.5%$ of the rest
mass energy may be dissipated by the gravitational wave emission in the final
phase of the merger. These results confirm that the new spatial gauge condition
is promising in many problems at least up to the formation of black holes. In
the case in which a black hole is formed, on the other hand, the gauge
condition seems to be less adequate, but we suggest a strategy to improve it in
this case. All of the results obtained confirm the robustness of our
formulation and the ability of our code for stable evolution of strong
gravitational fields of compact binaries.
