Simulations of a thermally unstable ISM model. Upper panels show slices of the density field for the Athena grid code (left) and the SPH code Fi (right) after 0.5 Myr of evolution. In both cases the thermal evolution is calculated using constant heating and the simple analytic Gerritsen & Icke (1997) cooling curve, resulting in a thermally unstable model. This can be seen in the (equilibrium) density-pressure relation (lower left panel). In the lower right panel we show the evolution of the cool (T < 100 K, blue lines) and dense (ρ > 10 cm^-3, red lines) gas fractions, where the solid lines show the results for Athena, dash-dotted those of Fi. Both models show broadly similar behaviour, although clearly differences can be seen, which must be traced to the use of different numerical hydrodynamic solvers.