Ab initio potential energy surface and near-infrared spectrum of the He–C2H2 complex

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Abstract
Symmetry‐adapted perturbation theory has been applied to compute the intermolecular potential energy surface of the He–C2H2 complex. The interaction energy is found to be dominated by the first‐order exchange contribution and the dispersion energy. In both contributions it was necessary to include high‐level intramolecular correlation effects. Our potential has a global minimum of εm=−22.292 cm−1 near the linear He–HCCH geometry at Rm=8.20 bohr and ϑm=14.16°, and a local minimum at a skew geometry (Rm=7.39 bohr, ϑm=48.82°, and εm=−21.983 cm−1). The computed potential energy surface has been analytically fitted and used in converged variational calculations to generate bound rovibrational states of the He–C2H2 molecule and the near‐infrared spectrum, which corresponds to the simultaneous excitation of the vibration and hindered rotation of the C2H2 monomer within the complex. The nature of the bound states and of the spectrum predicted from the ab initio potential are discussed.