The intermolecular potential of NH4+-Ar I. Calculations for the internal rotor structure of the nu(3) band

The intermolecular potential energy surface of the electronic ground state of the ammonium-argon ionic dimer, NH4+-Ar, is calculated by ab initio meth ods using different levels of theory (MP2, MP4) and basis sets (aug-cc-pVXZ , X = D/T/Q). The deformation of the ammonium ion in the complex is shown t o be small and its geometry is therefore fixed in these calculations to the tetrahedral structure optimized for the bare ion. The global minimum of th e potential corresponds to a proton-bound structure with C-3v symmetry (R-e approximate to 3.4 Angstrom, D-e approximate to 950 cm(-1)) and the barrie r to internal rotation between the four equivalent minima is around 200 cm( -1). The three-dimensional potential is expanded in tetrahedral harmonics w hose radially dependent coefficients, V-i(R), are compared for the consider ed levels of theory. The rotation-intermolecular vibration Hamiltonian is s olved using a two-dimensional fixed-Ii representation of the calculated pot entials, V-i drop V-i(R-eff), where the effective intermolecular separation , R-eff, is determined from the experimental rotational constants of the co mplex. The accuracy of these parametrized potential energy surfaces is judg ed by their ability to reproduce the hindered rotor subband structure in th e experimental nu(3)(t(2)) infrared band of the complex. The simulations us ing the potentials calculated at the MP2/aug-cc-pVTZ or higher levels of th eory reproduce the coarse structure of the experimental spectrum well. Furt her improvement could be achieved by least-squares fitting the potential pa rameters to the observed subband positions. The fitted V-3 and V-4 paramete rs remain in close agreement with those determined from the ab initio calcu lations but the anisotropy of the potential is significantly different from that in a previous least-squares fit V-3 alone.