Potential energy hypersurface for the 1 2Π(1 2 B 1,1 2 A ′′) Rydberg state of the HeH2+ system

The 1 2Π(1 2B1,1 2A″) state of the helium dihydride ion HeH2+ has been examined with ab initio electronic structure calculations. In particular, the potential energy hypersurface for this Rydberg state has been surveyed with an MCSCF/CI wavefunction involving orbital expansions in a double-zeta plus double polarization plus diffuse one-electron basis. The surface is essentially a trough falling monotonically from the atom–diatomic asymptote He(X 1S) +H2+(C 2Πu), through the triatomic region, to the asymptote HeH(1 2Π)+H+. At some triatomic geometries the 1 2Π state is one of the lowest excited electronic states of this molecular ion system. The results obtained here are the first reported explicit treatment of HeH2+ Rydberg states, and will contribute towards the elucidation of the manifolds of the lowest two Rydberg series arising from the (HeH,H)+ excited asymptotes.