Criterion describing, for a given ionization front velocity, the G₀∕n_H ratio leading to a merge of the ionization and dissociation fronts. The ionization front and the dissociation front are merged to the right of the criteria. The black dotted line is the criterion derived by Bertoldi & Draine (1996), the black dashed line is the one of Störzer & Hollenbach (1998). The red circles are the effective drop from Fig. 6. The black line is our new semi-analytical criterion from Eq. (15) and the blue dashed line is a fit of the criterion from Eq. (16).