Dependences of the chemical model on various free parameters. Top left: the fractional abundances of CH₃OH, C¹⁸O, and N₂H⁺ are shown for four different chemical ages. Same colors identify abundance curves at the same chemical time for chemical ages as indicated in the legend. Top right: the fractional abundances of C¹⁸O (dashed lines) and N₂H⁺ (full lines) are displayed for different values of the CO binding energy. Same colors identify abundance curves computed with the same CO binding energy as indicated in the legend. Bottom left: the fractional abundances of C¹⁸O (dashed lines) and N₂H⁺ (full lines) are displayed for different values of the initial fractional CO abundance. Same colors identify abundance curves computed with the same CO initial abundance as indicated in the legend. Bottom right: the fractional abundances of H₂O, CH₃OH, and C¹⁸O are shown for four different values of the initial fractional H₂O abundance. Same colors identify abundance curves computed with the same H₂O initial abundance as indicated in the legend.