  The Liouville equation is of fundamental importance in the derivation of
continuum models for physical systems which are approximated by interacting
particles. However, when particles undergo instantaneous interactions such as
collisions, the derivation of the Liouville equation must be adapted to exclude
non-physical particle positions, and include the effect of instantaneous
interactions. We present the weak formulation of the Liouville equation for
interacting particles with general particle dynamics and interactions, and
discuss the results using an example.
