Distributions of the logarithmic molecular abundance ratio of M1 and M2 for the gas-phase species and grain species as a function of their diffusion energy at the three different evolutionary times. These results are under the assumption that there is no difference when the abundance is lower than 10⁻¹⁴ for a species in the two models. Panel a shows model parameters with a grain temperature T_d = 10 K and E_dif/E_des = 0.5, and panel b shows T_d = 20 K and E_dif/E_des = 0.3. X_M1 and X_M2 represent the abundance of a species in models M1 and M2, respectively. The horizontal dashed lines plot the ±0.3 difference of the logarithmic molecular abundance ratio of M1 and M2.