Analytic three-dimensional ‘MLR’ potential energy surface for CO2–He, and its predicted microwave and infrared spectra

Abstract

A three-dimensional, analytic potential energy surface for CO₂–He that explicitly incorporates its dependence on the Q₃ asymmetric-stretch normal-mode coordinate of the CO₂ monomer has been obtained by least-squares fitting new ab initio interaction energies to a new three-dimensional Morse/Long-Range (3D-MLR) potential function form. This fit to 2832 points has a root-mean-square (RMS) deviation of 0.032 cm⁻¹ and requires only 55 parameters. The resulting pure ab initio potential provides a good representation of the experimental microwave and infrared data: for 51 pseudo microwave and 49 infrared transitions the RMS discrepancies are 0.0110 and 0.0445 cm⁻¹, respectively. Scaling this surface using only two morphing parameters yields an order of magnitude better agreement with experiments, with RMS discrepancies of only 0.0025 and 0.0038 cm⁻¹, respectively. The calculated infrared band origin shift associated with the ν₃ fundamental of CO₂ is 0.109 cm⁻¹, in good agreement with the (extrapolated) experimental value of 0.095 cm⁻¹.