Accurate Spectroscopic Models for Methane Polyads Derived from a Potential Energy Surface Using High-Order Contact Transformations

A new spectroscopic model is developed
for theoretical predictions
of vibration–rotation line positions and line intensities of
the methane molecule. Resonance coupling parameters of the effective
polyad Hamiltionians were obtained via high-order contact transformations
(CT) from ab initio potential energy surface. This allows converging
vibrational and rotational levels to the accuracy of best variational
calculations. Average discrepancy with centers of 100 reliably assigned
experimental bands up to the triacontad range was 0.74 cm–1 and 0.001 cm–1 for GS rotational levels up to J = 17 in direct CT calculations without adjustable parameters.
A subsequent “fine tuning” of the diagonal parameters
allows achieving experimental accuracy for about 5600 Dyad and Pentad
line positions, whereas all resonance coupling parameters were held
fixed to ab initio values. Dipole transition moment parameters were
determined from selected ab initio line strengths previously computed
from a dipole moment surface by variational method. New polyad model
allows generating a spectral line list for the Dyad and Pentad bands
with the accuracy ∼10–3 cm–1 for line positions combined with ab initio predictions for line
intensities. The overall integrated intensity agreement with Hitran-2008
empirical database is of 4.4% for the Dyad and of 1.8% for the Pentad
range.