starlight background. This rate is assumed to be a function of the radiation field scaled to the intensity of the Galactic background, and the visual extinction . In contrast we explicitly integrate stored cross sections over the local radiation field for those processes which have energy-specific data. These include photo rates for all atoms, ions, and grains, and some molecules, most notably . The incident radiation field is attenuated by the computed gas and dust opacity. Our treatment can handle any grain opacity distribution or abundance, or SED shape.

The grain scattering theory predicts the scattering asymmetry factor $g$, which is the average of the cosine of the scattering angle of an incident photon [@Martin1979]. When a point source like a star is viewed through an intervening cloud at a large distance from the star, photons scattered by even a small amount are lost from the beam and cannot reach the observer. So in this case scattering attenuates the radiation field by the full opacity, $\alpha_{scat}$. We refer to this, the quantity measured by observations of stellar extinction, as the point-source extinction. On the other hand, when modeling a cloud irradiated by a star, photons that are scattered in a 