Calculated abundances of different species relative to the observed column density of CCH, as a function of time for a constant density n(H₂) = 5 × 10⁵ cm^-3 and a warm-up to T_max = 30 K from 10 K, where the warm-up and other conditions are described in Sect. 6. The dashed horizontal lines correspond to the observed abundances and are color-coded with the calculated abundances. The vertical lines at t = 0.1 and 0.3 Myr indicate the onset and end of warm-up of grains, respectively.