which operate in the systems which are modeled. by @HenneyEtAl05 to treat the structure of  region photo-evaporation fronts, and extended by @Henney2007 to the molecular knots within planetary nebulae, including the PDR ahead of the  region.

Work has also gone into improving the robustness of handling of processes which couple different parts of the system sufficiently strongly that a simple iterative scheme was slowly convergent. The particular cases where this has been found to be an issue is in the handling of the resonant O/H charge transfer process [@Stancil.P99Charge-transfer-in-collisions-of-O-with], the rate of which can by far dominate direct ionization processes for either ion, and the handling of Rydberg levels which are more strongly coupled to the continuum than the base ion. A simple solution acceleration approach has been found to be sufficient to allow rapid convergence, details of which will be given in Williams et al (in preparation).

In addition, the solvers for the electron density and temperature have been completely rewritten and are now much more robust and reliable than the previous versions. These changes are especially important for modeling extreme environments: very cold regions on the one hand (PDRs and molecular regions) and very hot on the other 