Analytic Morse/long-range potential energy surfaces and “adiabatic-hindered-rotor” treatment for a symmetric top-linear molecule dimer: A case study of CH3F–H2

A first effective six-dimensional ab initio potential energy surface (PES) for CH3F–H2 which explicitly includes the intramolecular Q3 stretching normal mode of the CH3F monomer is presented. The electronic structure computations have been carried out at the explicitly correlated coupled cluster level of theory [CCSD(T)-F12a] with an augmented correlation-consistent triple zeta basis set. Five-dimensional analytical intermolecular PESs for ν3(CH3F) = 0 and 1 are then obtained by fitting the vibrationally averaged potentials to the Morse/Long-Range (MLR) potential function form. The MLR function form is applied to the nonlinear molecule-linear molecule case for the first time. These fits to 25 015 points have root-mean-square deviations of 0.74 cm−1 and 0.082 cm−1 for interaction energies less than 0.0 cm−1. Using the adiabatic hindered-rotor approximation, three-dimensional PESs for CH3F–paraH2 are generated from the 5D PESs over all possible orientations of the hydrogen monomer. The infrared and microwav...