Time evolution of the distribution of ζ₂ for the gas particles in the simulation for models ℋ (top) and ℒ (bottom). The distributions are determined from a box of 0.2 × 0.2 × 0.2 pc³, which only includes particles from the simulated cloud, i.e., with identical mass. The rightmost edge of the distribution shrinks over time as the gas particles within the box become denser. The low-end tail of the distribution expands towards lower ionisation rates, which is consistent with the evolution of the gas density.