Fractional abundances X(a) = n(a)/n_H (left panel) and fractional mass ρ(a)/n_H of grain bins for the cases listed in Table 1. The black line represents the initial (MRN) distribution, while the other lines give the distribution at χ = 8.12 × 10¹⁷ cgs. The right panel shows the relative error of the constant density cases nH0, nH4, and nH8 (indistinguishable to machine precision) compared to the simulation Sim.