Full quantum calculation of the rovibrational states and intensities for a symmetric top-linear molecule dimer:...

The rovibrational energy levels and intensities of the CH₃F-H₂ dimer have been obtained using our recent global intermolecular potential energy surface [X.-L. Zhang et al., J. Chem. Phys. 148, 124302 (2018)]. The Hamiltonian, basis set, and matrix elements are derived and given for a symmetric top-linear molecule complex. This approach to the generation of energy levels and wavefunctions can readily be utilized for studying the rovibrational spectra of other van der Waals complexes composed of a symmetric top molecule and a linear molecule, and may readily be extended to other complexes of nonlinear molecules and linear molecules. To confirm our method, the rovibrational levels of the H₂O-H₂ dimer have been computed and shown to be in good agreement with experiment and with previous theoretical results. The rovibrational Schrödinger equation has been solved using a Lanczos algorithm together with an uncoupled product basis set. As expected, dimers containing ortho-H₂ are more strongly bound than dimers containing para-H₂. Energies and wavefunctions of the discrete rovibrational levels of CH₃F-paraH₂ complexes obtained from the direct vibrationally averaged 5-dimensional potentials are in good agreement with the results of the reduced 3-dimensional adiabatic-hindered-rotor (AHR) approximation. Accurate calculations of the transition line strengths for the orthoCH₃F-paraH₂ complex are also carried out, and are consistent with results obtained using the AHR approximation. The microwave spectrum associated with the orthoCH₃F-orthoH₂ dimer has been predicted for the first time.