ExoMol line lists – LXX: The CO fourth positive band system

A high-temperature line list is provided for the 12C16O A1Π − X1Σ+ electronic band system, commonly referred to as the fourth positive system, the most extensively studied excited electronic system of carbon monoxide. New high-accuracy spectroscopic model for the ground X1Σ+ and excited A1Π states is presented, based on ab initio calculations performed at the aug-cc-pV5Z CASSCF/icMRCI+Q level of theory. The resulting potential energy curves and transition dipole moment functions were further refined using the duo program in conjunction with available marvel empirical energy levels to improve the accuracy of the final line list. Using the refined .states and .trans files, the comprehensive 4th+ line list contains CO fourth positive transitions and updated rotation-vibration transitions within the X1Σ+ ground state. Comparisons with several high-resolution experimental spectra demonstrate very good agreement in both line positions and band structures, confirming the reliability of the present spectroscopic model. The 4th+ line list is available from www.exomol.com.