and these should not be neglected [e.g., @vieitez_etal2008a; @lewis_etal2008a; @heays2011b].

![image](compare_cross_sections){width="\textwidth"}

The wavelengths of the majority of the modelled rovibrational lines are accurate to within 0.01Å when compared with laboratory measurements [@heays2011]. The magnitudes of the calculated cross sections are accurate to about 10%, with their principal uncertainty arising from the absolute calibration of the laboratory absorption oscillator strengths used to constrain the model electronic transition moments [@stark_etal2005; @stark_etal2008; @heays_etal2009]. The predissociation fractions pertaining to the longest wavelength bands are well known and modelled [@lewis_etal2005a] but are less certain at shorter wavelengths. For these, a predissociation fraction of 1 is assumed. This is a reliable assumption with respect to ${}^1\Pi_u$ states, many of which are known to exhibit significant predissociation broadening [@stark_etal2008; @heays2011b]. It is less certain that all ${}^1\Sigma^+_u$ levels responsible for absorption at shorter wavelengths are completely predissociative. Further practical details regarding the calculation of N$_2$ cross sections and dissociation fractions at different temperatures may be found in @li2013.

Also shown in Fig. \[fig:cross sections\] are transmission functions representing further line-absorbing species relevant to the following treatment of the shielding of N$_2$ absorption in astrophysical environments. For these, photoabsorption cross sections for H$_2$ are synthesised from data obtained 