Density cuts of four simulations of an ionization front propagation on clumps (0.5 parsec in diameter). The UV flux at the top of the box is 10⁹ photons per second per square centimetre, the box is 4 × 2 × 2 parsecs (half of the cut is shown, only the central parsec of the simulation box). The last simulation (case (d)) is an isolated spherical clump of constant density 1000 cm³, the three others are clumps in a homogeneous medium of density 500 cm³. The clumps have densities of 1000 for case (a), 1750 for case (b) and 2500 cm³ for case (d).