Temperature map as a function of r and t of the physical model calculated with the radiative transfer code  MOCASSIN, assuming T = T_d, and where the colour bar reports log(T). The chemical network is evolved in time at each radius, varying the dust and the gas temperature according to this model, while density is a function of r only. We selected three models at three specific radii (marked A, B, and C) to discuss the impact of the multiple binding energy approach, see Fig. 3. At these radii, we further discuss the distribution of the chemical abundances at specific (r, t) combinations, indicated by the circular markers and by the horizontal dashed lines (see Fig. 4). The inner (r ≲ 3 × 10³ au) high-temperature region of the envelope is less relevant for the overall discussion, given the relatively short evaporation timescale, and for this reason it is ignored in our discussion.