Theoretical study of ion-molecule potentials for He+ and Li+ with N2

High-level ab initio calculations have been carried out on the lowest charge-transfer excited state of [HeN2](+) and the [LiN2](+) ground state, over a range of intermolecular distances R of 3 - 2a(0) at a fixed N-2 bond length r(e) = 2.074 30a(0) for three orientation angles gamma = 0 degrees, 45 degrees, and 90 degrees. The calculations employed extended atomic basis sets, chosen to represent accurately the electrical properties of the interacting partners; for N-2 the key properties alpha(parallel to), alpha(perpendicular to), and Theta are within 1.7%, 2.7%, and 2.1% of the best experimental values. All interaction energies were corrected for basis-set superposition error by the counterpoise method, and fitted by analytic forms incorporating the proper long-range expansion through R-7. Our value for the tetrahexacontapole (2(6)-pole) moment of N-2 is -15.95 a.u. The most stable geometries for both systems occur for linear (gamma = 0 degrees) complexes, with minima -D-e of -7.00 (-12.65) kcal/mol located at R-e = 3.048 (2.610) Angstrom for He+(Li+) + N-2; the Li+ values are in good agreement with previous theoretical results. At small R the splitting Delta V(R, gamma) between the He+ and Li+ surfaces is found to have a radial dependence close to the prediction of the hydrogenic charge-transfer coupling model, while its anisotropy is similar to that of the surfaces themselves. (C) 1998 American Institute of Physics. [S0021-9606(98)30740-0].