Upper panel: the abundances (with respect to n_H, left-hand scale) of the cooling molecules (indicated in the figure) as a function of core radius in model B. Solid lines correspond to the initial chemistry calculation with T_gas = T_dust, other linestyles to subsequent iterations with T_gas ≠ T_dust. The black curves represent T_gas at different iterations (right-hand scale). Lower panel: the abundances of the cooling molecules and of OH as a function of time in a single-point chemical model with n_H = 2.0 × 10⁵ cm^-3 and T_gas = T_dust = 9 K.