The Fe(IV) d-d transition energies for four active-site structural models of class I ribonucleotide reductase (RNR) intermediate X have been calculated using broken-symmetry density functional theory incorporated with the Slater transition state vertical self-consistent reaction field methodology. Our model I ( Figure 1), which contains two mu-oxo bridges, one terminal water, and one bidentate carboxylate group, yields the best Fe( IV) d-d transition energies compared with experiment. Our previous study (J. Am. Chem. Soc. 2005, 127, 15778-15790) also shows that most of the other calculated properties of model I in both native and mutant Y122F forms, including geometries, spin states, pK(a)'s, Fe-57, H-1, and O-17 hyperfine tensors, and Fe-57 Mossbauer isomer shifts and quadrupole splittings, are also the best in agreement with the available experimental data. This model is likely to represent the active-site structure of the intermediate state X of RNR.