Spin-state abundance ratios of the various isotopologs as functions of time. The medium density is n(H₂) = 10⁶ cm^-3 (upper row) or n(H₂) = 10⁷ cm^-3 (lower row). From left to right, the panels show calculations assuming T_gas = T_dust = 10 , 15, or 20 K. Species-to-species rate coefficients are adopted in two of the models (method 1, dashed lines; method 2, solid lines). The dotted lines show the results of calculations using the ground-state-to-species rate coefficients.