Physical structure of the Horsehead nebula with the density fixed for the dense part of the PDR. All panels are represented as functions of the visual extinction, A_V. The first panel shows the density and position profiles, while the second panel represents the molecular hydrogen fraction f(H₂) computed by both codes, with the solid line indicating the Nautilus code and the dashed line the Meudon PDR code. The third panel represents the grain- and gas-temperature profiles. The green area covers the range of dust temperature given by the Meudon PDR code for different grain size values in the range 0.03 to 0.3 μm.