Schematic view of the mass fluxes in a multi-phase gas particle. Gas cooling moves mass from the hot to the cold phase at a rate Ṁ_cool. Within the cold phase, an H₂ reservoir begins to grow. ${\dot{M}}_{\mathrm{form}{\mathrm{H}}_2}$ is the H₂ formation rate determined by the particle dust content and the number density of atomic hydrogen, which in turn is computed from the cold phase mass. ${\dot{M}}_{\mathrm{dest}{\mathrm{H}}_2}$ is the H₂ destruction rate by LW photons, which transports mass from the molecular reservoir back to the cold phase. Ṁ_sf is the rate at which molecular gas creates a virtual stellar phase, which is contained within the gas particle until a stellar particle is stochastically spawned. Further destruction of the molecular mass occurs due to the action of massive stars and AGN, at rates Ṁ_ev and ${\mathrm{f}}_{\mathrm{H}2}{\dot{M}}_{\mathrm{AGN}}$, respectively, where f_H2 represents the fraction of H₂ in the cold phase. AGN also moves material from the cold to the hot phase at a rate Ṁ_AGN