is different from most heavy elements, with a first excited level at (1-1/$n^2$) = 0.75 of the ionization energy and more highly excited levels close to the ionization limit. By contrast, the heavy elements can have many low-lying levels. This means that, for most temperatures, lines from H and He – like species will have a strong recombination component, while lines of many-electron systems will be predominantly collisionally excited. The structure of the H and He – like iso sequences also means that a significant number of excited states are needed for the model atoms to correctly go to LTE in the high particle or photon limits.

Steve Cota developed the original model of , , and  emission in   as part of his PhD thesis [@1987PhDT.........7C]. He also developed an approximate treatment of three-body recombination for the heavy elements, described in the next section. This was extended by Jason Ferguson in his PhD thesis [@Ferguson.J97Spectral-Simulations-and-Abundance-Determinations; @FergusonFerland1997] to include more levels, as processor power increased. These models, which involved 15 levels with a number of higher pseudo states, were significant time sinks on the computers at that time.

Today’s unified model of the H and He iso-electronic sequences was developed as 