Central diffusion coefficient η_eff versus number density for different grain sizes, extracted from the dynamical model. The vertical line at the left indicates the density at which the detailed chemistry and non-ideal MHD treatment is switched on. Beyond n_c ≈ 10¹⁸ cm^-3 the resistivity plummets, after having already declined due to thermal ionization. This is where we switch the chemistry calculations off again, and is denoted by the vertical line on the right. Due to grain destruction, flux-freezing is restored there.