Geometries and frequencies for the neutral MX2 and ionic MX2-species (M=L1, Na, and X=F, Cl) 2 are studied by several theoretical methods: density functional theory (Becke-3-Lee-Yang-Parr) [DFT(B3LYP)], second-order many-body perturbation theory [MBPT(2)], and coupled-cluster with singles and doubles (CCSD). The geometries optimized at the CCSD/6-311+G(d) level are used in CCSD(T) calculations with a large atomic natural orbital basis to compute adiabatic electron o affinities (EA(ad)), which are found for LiF2, LiCl2, NaF2, and NaCl2 to be 5.45, 4.97, 5.12, and 4.69 eV, respectively. The highest EAs among all the atoms of the periodic table occur in the halogen atoms (fluorine, 3.40 eV; chlorine, 3.62 eV); therefore all four of these triatomic radicals are properly termed superhalogens. LiF2, LiCl2, NaF2, and NaCl2 are thermodynamically stable, and their dissociation energies computed at the CCSD with the noniterative inclusion of triples [CCSD(T)] level are 20.5, 24.9, 19.3, and 25.2 kcal/mol, respectively. LiF2-, LiCl2-, NaF2-, and NaCl2- are more stable than their neutral parents with CCSD(T) dissociation energies of 69.5, 58.7, 49.0, and 52.5 kcal/mol, respectively. The computed vertical electron detachment energies of LiF2-, LiCl2-, NaF2-, and NaCl2- are 6.51, 5.88, 6.18, and 5.77 eV, respectively, which are in nice agreement with the values calculated by Scheller and Cederbaum by the Green-Function method. (C) 1997 American Institute of Physics.