ζ₂ values (divided by R_H × n(H₂)) obtained with Eq. (B.6) at 15 K, as a function of R_D and f_D. The bottom-right corner is missing because it violates the condition R_D < 0.029 × f_D. The plot shows that the quantity ζ₂/(R_H × n(H₂)) increases by up to four orders of magnitude when the deuteration fraction decreases from 0.1 to 10⁻³ and that the CO depletion factor plays a significant role at R_D typical for dense gas (0.01 – 0.1).