2024MNRAS.528.3719Y

The MM ro-vibrational line list for methane (¹²CH₄) is presented; MM covers wavelengths λ > 0.83 μm (wavenumbers up to 12 000 cm^–1) and contains over 50 billion transitions between 9155 208 states with total angular momentum J ≤ 60. MM was generated through solution of the nuclear motion Schrödinger equation using variational program TROVE for an empirically derived potential energy surface (PES) and a new high-level ab initio dipole moment surface. The PES was constructed by fitting the ro-vibrational energies of CH₄ to a set of highly accurate, experimentally derived energies. Molecular states are classified using the T_d(M) symmetry group and are fully assigned with rotation and vibration quantum numbers. The MM line list is adapted to high-resolution applications by replacing the calculated ro-vibrational energies with the experimentally derived values where available, namely for 23 208 states with J ≤ 27 below 9986 cm^–1. Doing so leads to over 1000 000 experimentally derived CH₄ lines compared to approximately $330\, 000$ lines of ¹²CH₄ in the HITRAN data base. The MM line list is shown to be more complete than the recent HITEMP methane line list. Methane spectra computed using MM across a broad range of temperatures and wavenumbers show excellent agreement with experiment. The MM line list supersedes the previous ExoMol methane line lists 10to10 and 30to10 both in terms of accuracy and coverage. Together with the pre-computed ExoMolOP molecular atmospheric opacity tables, it is now the recommended CH₄ data set in the ExoMol data base (www.exomol.com).