Evolution of the modelled gas parameters, Z, n_H, and N_H, as a function of depth (A_V) for a solar-metallicity cloud (top left) and for a cloud of 0.1 Z_⊙ (top right), with a starting density of 10³ cm⁻³ and r_in of 10^20.7 cm, G₀ = 380 in terms of the Habing field. Two central panels: effect of density variations (left: n_H = 10² cm⁻³; right: n_H = 10⁴ cm⁻³) for Z = 0.5 Z_⊙ with G₀ = 380. Bottom two panels: effect of G₀ variations (left: G₀ = 80; right: G₀ = 1800) for constant Z (0.5 Z_⊙) and constant n_H (10³ cm⁻³). Each panel contains subpanels within, from top to bottom: cumulative mass in the ionised, neutral atomic, and molecular phases; normalised cumulative intensity of the [C II]λ158 μm, [C I]λ610 μm, and CO (1−0) lines; abundance of C⁺, C⁰, and CO relative to C; hydrogen density (n_H) as a function of column density (N_H) and A_V; fraction of hydrogen in the ionised, atomic, and molecular form. The vertical dashed lines indicate the depth of the main phase transitions: H⁺ to H⁰, H⁰ to H₂, CO(1−0) optical depth of 1. Masses indicated here should be adjusted for individual galaxies, scaling as L_TIR (galaxy)/10⁹ L_⊙, as the source luminosity of the model is 10⁹ L_⊙ (see Appendix A).