Volume cooling rate defined in Eq. (1) for a hydrogen-nuclei number density of 1 cm^-3 as a function of temperature for different H₂ to total hydrogen mass ratios f_H2. For high fractions of H₂ and at such densities, the cooling can become slightly more important just below 10⁴ K when the H₂ mass fraction is reduced, because the contribution of collisions with atomic hydrogen dominates in this range of temperature.