electronic lines of  are especially important sources of higher-energy photons.

Molecular excitations and spectra
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We next describe our treatment of molecular emission processes. Gargi Shaw developed our model of the  molecule, the most common molecule in the Universe, as part of her PhD thesis [@Shaw2005]. All levels within the ground electronic state are included [@Dabrowski.I84The-Lyman-and-Werner-bands-of-H2], along with all electronic excited states on the Meudon web site[^4] and @Abgrall.H94The-Bprime1Sigmau-rarr-X1Sigmag-and-D1Piu. Collision rates are from @Wrathmall2007. @Lee.T05Close-coupling-calculations-of-low-energy-inelastic [@Lee.T06State-to-state-rotational-transitions-in-H2H2; @LeeH2H22008], @Shaw.G09Rotationally-Warm-Molecular-Hydrogen, and @Gay.C12Rovibrationally-Resolved-Direct-Photodissociation summarize more recent updates.

Grain formation pumping of  can be treated using several theories. @Shaw2005 provides more details. We use the @Takahashi2001 results by default.

Line emission for molecules heavier than  is predicted using the level energy, collision, and radiative data in the LAMDA database [@Schoier.F05An-atomic-and-molecular-database-for-analysis]. The molecular excitation are solved simultaneously and self-consistently with the global environment. This includes grain properties, including emission, so molecular pumping in the infrared continuum is automatically included (if the pumping lines are present in the molecular data), along with attenuation by grains and other continuous opacity sources. Line optical depths are computed for each point in the cloud, and the full radiative transfer performed as described below.

Grains
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Grains have been included 