Convergence of normal mode variational calculations of methane spectra: Theoretical linelist in the icosad range computed from potential energy and dipole moment surfaces

Accurate basis set convergence of first-principles predictions of rotationally resolved spectra at high energy range is a common challenging issue for variational methods. In this paper, a detailed convergence study for the methane spectra is presented both for vibrational and rotational degrees of freedom as well as for intensities. For this purpose, we use our previously reported nine-dimensional potential energy and dipole moment surfaces of the methane molecule [Nikitin et al. Chem Phys Lett 2011;501:179–86; 2013;565:5–11]. Vibration–rotation calculations were carried out using variational normal mode approach with a full account of the Td symmetry. The aim was to obtain accurate theoretical methane line lists for the wavenumber range beyond currently available spectra analyses. The focus of this paper is the complicated icosad range (6280–7900 cm−1) containing 20 bands and 134 sub-bands where over 90% of experimental lines still remain unassigned. We provide variational line lists converged to a “spectroscopic precision” for icosad transitions for T=80 K and T=296 K. The first one contains 70 300 lines and the second one 286 170 lines with the intensity cut-off 10 − 29 cm − 1 / ( molecule cm − 2 ) with Jmax=18. An average error in line positions of theoretical predictions up to J=15 is estimated as 0.2–0.5 cm−1 from the comparisons with currently analyzed bands. Ab initio line strength calculations give the integrated intensity 4.37 × 10 − 20 cm − 1 / ( molecule cm − 2 ) at T=80 K for the sum of all icosad bands. This is to be compared to the integrated intensity 4.36 × 10 − 20 cm − 1 / ( molecule cm − 2 ) of the experimental icosad line list recorded in Grenoble University [Campargue et al., J Mol Spectrosc 2013;291:16–22] using very sensitive laser techniques. The shapes of absorption bands are also in a good qualitative agreement with experimental spectra.