Double-Rydberg anions: Ground-state electronic and geometric stabilities

Each member of the class of Double-Rydberg (DR) molecular anions consists of an underlying closed-shell cation core around which a pair of highly correlated electrons move in diffuse orbitals. We have examined the geometric and electronic stabilities of the ground states of candidate DR anions resulting from the following cation cores: H⁺₃, NeH⁺, FH⁺₂, H₃O⁺, NH⁺₄, and CH⁺₅ . Near the equilibrium geometry of the cation, all of the DR anions, except H^-₃, are electronically stable with respect to the corresponding Rydberg radicals. Results of our geometry optimizations indicate, however, that only NH^-₄ and H₃O^- are locally geometrically stable; the other DR anions undergo fragmentation. Vertical ionization potentials for the T_d isomer of NH^-₄ and the C_3v isomer of H₃O^- are found to be 0.45 and 0.46 eV, respectively.