The electronic and the molecular structures of Fe[subscript 2] Fe[subscript 2][superscript +] Fe[subscript 2][superscript ++] and Fe[subscript 2][superscript -] were investigated using the multiconfiguration self consistent-field (MCSCF) and mul-tireference configuration interaction (MRCI) methods with various basis sets. For Fe[subscript 2], the ground state is the [superscript 7] delta [subscript u] with sigma[superscript 4] pi[superscript 6] gamma[superscript 6] electronic structure, the structure which is markedly different from the previous studies. The equilibrium nuclear distance (R[subscript e]) of 4.15 Bohr and the zero-point frequency (omega[subscript e]) of 215.0 cm[superscript 1] were obtained. This is in good agreement with the experimental R[subscript e] and omega[subscript e] of 3.53 +-0.24 or 3.82 +-0.04 Bohr and 299.6 cm[superscript 1], respectively. For Fe[subscript 2][superscript +], the ground state is the [superscript 8]delta[subscript u] with R[subscript e] and omega[subscript e] of 4.49 Bohr and 159.6 cm[superscript 1], respectively. For Fe[subscript 2][superscript -], the ground state is [superscript 8]delta[subscript 9] state with R[subscript e] and omega[subscript e] 4.03 (3.89-4.04) Bohr and 278.2 (250+-20) cm[superscript 1], respectively (The experimental values are given in parentheses). For Fe[subscript 2][superscript ++], among 4 states investigated the [superscript 9]pi[subscript 9] is the ground state with the energy around 30 kcal/mol below the next lowest state. The R[subscript e] and omega[subscript e] for this state are 5.16 Bohr and 176.5 cm[superscript 1], respectively. In this study, the first and the second ionizatios of Fe[subscript 2] are 5.1 and 15.9 eV, respectively while the experimental value is 6.30+-0.01 eV. The calculated electron affinity of 0.316 eV is underestimated in comparison with the experimental value of 0.902 +-0.008 eV. The calculations also showed that it is easier for Fe[subscript 2] to accept than to lose electron.
