Cationic van-der-waals complexes: Theoretical study of Ar2H+ structure and stability

The electronic and geometric structure, stability and molecular properties of the cationic van-der-Waals complex Ar 2 H + in its ground electronic state are studied by means of two ab-initio quantum-chemical approaches: conventional configuration interaction (multi-reference and coupled-cluster methods) and a diatomics-in-molecules model with ab-initio input data. To ensure consistency between the two approaches, one and the same one-electron atomic basis set (aug-cc-pVTZ by Dunning) is employed in both. The topography of the ground-state potential-energy surface is examined with respect to the nature of the binding and the stability of structures corresponding to stationary points. In accordance with most earlier theoretical work, there are two local minima at linear arrangements: a strongly bound centro-symmetric moiety, (Ar-H-Ar) + , and a weakly bound van-der-Waals complex, Ar...ArR + . These are separated by a low barrier. Only the centro-symmetric molecule is significantly stable (D e =0.68eV) against fragmentation into Ar+ArH + and should have structural and dynamical relevance. A fairly simple diatomics-in-molecules model taking into account only the few lowest electronic fragment states yields a qualitatively correct description of the ground state but shows quantitative deviations from the more accurate configuration-interaction data in detail. Nevertheless, it should provide a good starting point for the treatment of larger complexes Ar n H + with n > 2.