The electronic structure of transition metal dihelide dications

Multi-reference configuration interaction (MRCI) calculations have been employed to characterize the low-lying states of first-row transition metal dihelide dications, He₂TM²⁺ (TM = Sc–Cu). The most important state-ordering principles were determined to be the occupation of the 4s orbital and orientation of the occupied 3d orbital. The ground states of all species are predicted to be of D∞h symmetry arising from a 3dn⁺¹ electronic configuration. For excited states with singly occupied 4s or doubly occupied 3dσ orbitals, bending to C₂v symmetry typically lowers the energy and shortens the He–TM bond length. Coupled cluster singles and doubles with a perturbative treatment of triple excitations (CCSD(T)) results for ground state spectroscopic properties are in agreement with the MRCI predicted trends