Potential energy hypersurface for the 1 2Pi(1 2B1,1 2A'') Rydberg state of the HeH2+ system

The 1 2Pi(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 2Pi/sub u/), through the triatomic region, to the asymptote HeH(1 2Pi)+H+. At some triatomic geometries the 1 2Pi 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