THE LONG-RANGE INTERACTION BETWEEN HE AND H-2(- AN AB-INITIO AND DYNAMICAL STUDY() )

A multi-configuration self-consistent field, multi-reference configura tion interaction (MCSCF-MRCI) calculation with extended atomic basis s ets has been carried out on the electronic ground state of HeH2+ for i ntermediate-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 lengt h 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)) a t an intermolecular distance of 2.98 a(0) (1.576 Angstrom), in good ag reement with three previous high level calculations. The anisotropy of the interaction is strong, and is accounted for largely by the induce d-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 v ariationally using a basis set method. The intermolecular zero-point e nergy of the complex was found to be 1020 cm(-1), yielding a dissociat ion energy of 1484 cm(-1) (4.24 kcal mol(-1)). The influence of the an isotropy on complex formation and ion-molecule reactions at thermal en ergies was assessed using close-coupling optical potential calculation s of capture cross-sections; significant positive deviations from Lang evin 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.