the compute time required. The default treatment includes the greatest number of levels for H, and increasingly smaller numbers of levels for He, common second row elements like C and O, Fe, and the remaining low abundance elements.

Structure of other ions
-----------------------

 includes all ions of the lightest thirty elements. @Lykins.M12Radiative-cooling-in-collisionally-and-photo summarize recent developments in the treatment of the ions that are not part of the H and He iso-sequences.

An equivalent two-level system is assumed in modeling the ionization balance of ions of the Li-like and multi-electron iso sequences. Photoionization and collisional ionization from the ground configuration is balanced by recombinations to all levels. This assumes that nearly all populations are in ground, an approximation which is valid for moderate densities and the low temperatures usually found in photoionization equilibrium. Photoionization cross sections are from the Verner database [@VernerFerlandKorista1996] while radiative and dielectronic recombination rates are from the Badnell web site[^2] as described in @BadnellEtAl03 and @Badnell06, supplemented by data calculated as described by @VernerFerland1996. Charge exchange ionization and recombination rates are taken from an updated version of the @Kingdon1996 database.

The treatment of inner-shell processes, including line emission following removal of an inner-shell electron, largely follows F98. 