ExoMol line lists III: An improved hot rotation-vibration line list for HCN and HNC

A revised rotation-vibration line list for the combined hydrogen cyanide (HCN) / hydrogen isocyanide (HNC) system is presented. The line list uses {\it ab initio} transition intensities calculated previously (Harris et al., ApJ, 2002, 578, 657) and extensive datasets of recently measured experimental energy levels (Mellau, J. Chem. Phys. and J. Mol. Spectrosc. 2010-2011). The resulting line list has significantly more accurate wavelengths than previous ones for these systems. An improved value for the separation between HCN and HNC is adopted leading to an approximately 25%\ lower predicted thermal population of HNC as a function of temperature in the key 2000 to 3000 K region. Temperature-dependent partition functions and equilibrium constants are presented. The line lists are validated by comparison with laboratory spectra and are presented in full as supplementary data to the article and at \url{www.exomol.com}.