  Until recently the dynamical evolution of the interstellar medium (ISM) was
simulated using collisional ionization equilibrium (CIE) conditions. However,
the ISM is a dynamical system, in which the plasma is naturally driven out of
equilibrium due to atomic and dynamic processes operating on different
timescales. A step forward in the field comprises a multi-fluid approach taking
into account the joint thermal and dynamical evolutions of the ISM gas.
