terms are populated according to their statistical weight within the $n$ configuration [@PengellySeaton1964]. Such higher levels are treated as “collapsed levels” which are $nl$ mixed.

![The iso-sequence atomic level structure consists of low-lying $nl$ resolved ($nls$ for He-like) terms with $l-$mixed ($ls$ for He-like) collapsed configurations. The number of resolved and collapsed levels can be specified when the simulation parameters are established. \[fig:LevelsResolvedCollapsed\]](LevelsResolvedCollapsed){width="\linewidth"}

This treatment includes ions of all elements up to Zinc. The models include photoionization / recombination, collisional ionization / 3-body recombination, to all levels, and collisional and radiative processes between levels, so behave correctly in the low density limit and go to LTE at high densities or exposed to a true blackbody radiation field [@Ferland1988]. Line trapping, collisions, continuum lowering, and absorption of photons by continuous opacities, are all included as general processes [@Rees1989].

The user can adjust the number of resolved and collapsed levels modeled when the simulation is specified. The spectrum is predicted with great precision, an accuracy of better than 1%, when a larger number of levels are used [@Porter.R12Improved-He-I-emissivities-in-the-case-B-approximation]. This comes at the cost of increased execution times. Smaller models are often used for simulations of clouds with significant column densities due to 