into account. For instance, thermal grain infrared emission can fluoresce atoms or molecules within the cloud, and grain opacity impedes the propagation of emission lines out of the cloud.

As part of the maintenance and improvement of the underlying atomic database we have incorporated the UTA data computed by @Kisielius2003 and updated by Ferland et al. (2013, ApJ submitted). The radiative rates are very large and the corresponding radiative damping parameters can be substantially greater than 1. To improve the accuracy of predictions for such lines with high damping parameters, the Voigt function used in line transfer calculations now uses the fast and accurate implementation of @Wells1999, with some modifications to improve performance in the limit of small damping using the theory presented by @Hjerting1938.

The code includes a number of hyperfine structure lines [@Goddard.W03Hyperfine-Structure-Emission-and-Absorption]. We have put a major emphasis into the physics of the  21 cm line [@Shaw.G05Role-of-molecular-hydrogen-in-star-forming]. The line is usually optically thick, and pumping by   is treated as in @Deguchi1985. @Pellegrini2007 show how feedback between the stellar SED and fluorescent excitation of   alters the 21 cm optical depth and spin temperature in the M17  region.

Solution of material state
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Cloudy solves for self-consistent , and 