ExoMol line lists – LXII. Ro-vibrational energy levels and line strengths for the propadienediylidene (C3) in its ground electronic state

ABSTRACT Improved opacities are needed for modelling the atmospheres and evolution of cool carbon-rich stars and extra-solar planets; in particular, contributions made by the astrophysically important propadienediylidene (${\mathrm{C}}_{3}$) molecule need, at a minimum, to be determined using a line list which includes all significant transitions in the energy range of interest. We report variational calculations giving ro-vibrational energy levels and corresponding line strengths for $^{12}{\mathrm{C}}_3$, $^{12}{\mathrm{C}}^{13}{\mathrm{C}}^{12}{\mathrm{C}}$, and $^{12}{\mathrm{C}}^{12}{\mathrm{C}}^{13}{\mathrm{C}}$. In the $^{12}{\mathrm{C}}_3$ case, we obtain 2166 503 ro-vibrational state energies $\leqslant$2000 cm−1 for the electronic $\tilde{X}{\, }^{1}{\Sigma _{\rm g}}^{+}$ ground state. Comparison with experiment indicates a maximum error of $\pm 0.03$ ${\rm cm}^{-1}$ in calculated positions of lines involving an upper state energy $\lessapprox$4000 cm−1. For lines with upper state energies $\gtrapprox$4000 cm−1 to have comparable line-position accuracies, conical intersections would need to be accounted for in an adopted potential energy surface. Line lists and associated opacities are provided in the ExoMol data base (http://www.exomol.com).