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The atomic rates for H and He-like ions are accurate for densities up to $10^{18}$ cm$^{-3}$ by taking into account the effects of 3-body recombination and lowering of the continuum ([Bautista et al. 1998], [Bautista et al. 1999]). However, these effects are not treated for other species which reduce the accuracy of the model results for these ions at high density. The density for which these effects become important for a given ion increases rapidly with effective charge of the ion, starting at about $10^{12}$ cm$^{-3}$ for z=1 and at about $10^{16}$ cm$^{-3}$ for z=8. At low densities numerical errors can occur; the user is urged to read chapter 6.

Tim Kallman 2018-12-14