Anharmonic Vibrational Structure and Bound States of the He–CO2 van der Waals Complex

Abstract We present a new flexible-monomer two-body ab initio potential energy surface for the He–CO2 van der Waals complex, extending our studies of the Ne–CO2, Ar–CO2, and Kr–CO2 complexes to the least polarizable and most weakly bound member of the series. The shallow, anisotropic interaction makes the intermolecular motion strongly anharmonic and delocalized, such that local-mode excitation energies cannot necessarily be identified with physically bound vibrational levels. Curvilinear vibrational self-consistent field (VSCF) and vibrational configuration interaction (VCI) calculations are therefore used primarily to characterize intramolecular anharmonicity and mode coupling, while the floppy intermolecular states are treated quantitatively using coupled-channel BOUND calculations on vibrationally averaged effective potentials. The bound-state calculations yield a dissociation energy of D0 = 16.106 cm–1 and four excited J = 0 levels between 7.699 and 15.001 cm–1 relative to the ground state. In contrast, the VCI-derived local-mode stretching fundamental and the reported overtone and combination features lie above the dissociation threshold and should not be interpreted as ordinary bound levels. The intramolecular spectrum remains largely monomer-like: the VCI ν3 band origin is in close agreement with the experiment, the characteristic CO2 Fermi-resonance structure is retained, and the intramolecular bending degeneracy is predicted to be lifted by only ∼0.03 cm–1. Both the direct and bound-state treatments predict the experimentally observed positive sign of the complexation-induced ν3 shift, although its magnitude remains sensitive to the vibrational treatment. These results ultimately show that the low-energy spectrum of He–CO2 is governed by large-amplitude motion near dissociation, thereby outlining the limits of local-mode descriptions for weakly bound complexes.