Prediction of the column densities of C⁺, C, CO, CH, CCH  and c-C₃H₂  from the PDR Meudon models with various hydrogen densities and pressures. The constant density models are plotted with squares while the constant pressure models are shown with stars. These models correspond to A_V = 3   mag. Results from models with A_V = 5   mag are scaled by 0.6 to compensate for the higher total gas content, they are shown with open squares and dotted lines. Results from models with A_V = 1   mag are scaled by 3 to compensate for the lower total gas content, they are shown with crosses. The constant pressure models are plotted using the highest density point as abscissa.