We present results from molecular dynamics simulations of shock-induced hydrodynamic void collapse in a model energetic crystal. During void collapse, hotspot formation is observed that leads to subsequent detonation. The hotspot formation mechanism is identified as shock energy focusing via jetting. There is another initiation mechanism that arises from the interaction of reflected shock waves with the rigid piston, which is considered to be an artifact. Such artifact can be eliminated by altering the location of the void. The detonation threshold as a function of the velocity of the driven piston is determined for various void geometries. It is found that a system containing a void has a lower detonation threshold than that of a perfect energetic crystal. The amount of reduction of the detonation threshold depends on the geometry of the void. For square voids, there exists a minimum size above which reduction of the detonation threshold occurs. Among voids that have an equal volume, the void that is elongated along the shock direction gives the lowest detonation threshold.