Ground and first-excited global potential energy surfaces of the H ₂O⁺-He complex: Predictions of ion mobilities

Ion mobilities of H₂O⁺ drifting in helium are calculated and compared with experiment. These calculations employ global potential energy surfaces of the H₂O⁺-He complex, which in the present case were calculated ab initio at the unrestricted MP2 level of theory using a basis set of aug-cc-pVTZ quality, and treating the ion as a rigid body. Details are presented of the general characteristics of both the ground and first-excited electronic states of the complex. Although only the groundstate surface was used for the mobility calculations, the ab initio determination of the ground state necessitated the inclusion of the first-excited state owing to the presence of a crossing between the two. This crossing is also described. Mobilities calculated from the global surfaces are in good agreement with experiment.