  Liouville's theorem in a grand ensemble, that is for situations where a
system is in equilibrium with a reservoir of energy and particles, is a subject
that, to our knowledge, has not been explicitly treated in literature related
to molecular simulation. Instead Liouville's theorem, a central concept for the
correct employment of molecular simulation techniques, is implicitly considered
only within the framework of systems where the total number of particles is
fixed. However the pressing demand of applied science in treating open systems
leads to the question of the existence and possible exact formulation of
Liouville's theorem when the number of particles changes during the dynamical
evolution of the system. The intention of this note is to stimulate a debate
about this crucial issue for molecular simulation.
