Criterion describing, for a given ionization front velocity, the G₀∕n_H ratio leading to a merge of the ionization and dissociation fronts. The red circles are the empirical merging values. The black lines are our new semi-analytical criterion from Eq. (15) and the blue dashed lines are the fits from Eq. (18). The ionization front and the dissociation front are merged to the right of the criteria.