The long-range interaction between He and H-2(+): An ab initio and dynamical study

A multi-configuration self-consistent field, multi-reference configuration interaction (MCSCF-MRCI) calculation with extended atomic basis sets has been carried out on the electronic ground state of HeH2+ for intermediate-to-large intermolecular distances R between He and H-2(+) for both C-infinity v and C-2v geometries at a fixed H-2(+) bond length of 2.0 a(0). All energies were corrected for basis set superposition error using the counterpoise method. The most stable geometry occurs in C-infinity v with a well depth of 7.16 kcal mol(-1) (2504 cm(-1)) at an intermolecular distance of 2.98 a(0) (1.576 Angstrom), in good agreement with three previous high level calculations. The anisotropy of the interaction is strong, and is accounted for largely by the induced-dipole-quadrupole long-range force. The computed energy points were fitted to an analytical two-term Legendre expansion incorporating the appropriate long-range behaviour. This potential function was used to compute bound-state rovibrational energy levels of the HeH2+ complex variationally using a basis set method. The intermolecular zero-point energy of the complex was found to be 1020 cm(-1), yielding a dissociation energy of 1484 cm(-1) (4.24 kcal mol(-1)). The influence of the anisotropy on complex formation and ion-molecule reactions at thermal energies was assessed using close-coupling optical potential calculations of capture cross-sections; significant positive deviations from Langevin behaviour were found at sub-eV energies. The surface should be of value in interpreting the thermal energy Penning ionization of H-2 by excited He.