Notes. We are considering simple self-gravitating clouds where M_vir = (R(ΔV)²/G (virial mass M_vir, radius R and line width ΔV). We assume M_vir = 10 M_⊙ and temperature T_k = 50 K for every cloud. For model 1. N(H₂) = 2.5 × 10²¹ cm^-2, for 2. N(H₂) = 10²² cm^-2, and for 3. N(H₂) = 5 × 10²² cm^-2. X is the HCN abundance relative to H₂, N(HCN) the HCN column density and L is the HCN 3−2 luminosity: πR²I(HCN) ≈πR²T_B(HCN)1.06ΔV. The model clouds fulfil: HCN 3−2/1−0 = 0.3 to 0.4 and CO/HCN 1−0 = 1 to 3. The three HCN 1−0 hyperfine structures are summed in the intensity of the HCN 1−0 line since the inferred line width per model cloud is smaller than the velocity separation between the hyperfine structures. (We do not account for the radiative transfer effects in the ensemble of clouds which would lower the resulting HCN 1–0 intensity in relation to HCN 3–2 and CO 1–0). CO/HCN 1–0 ratios of unity are allowed to account for the possibility that not all of the CO flux is emerging from these HCN-emitting clouds.