Several models for the active site structure of class I ribonucleotide reductase (RNR) intermediate X have been studied in the work described in this paper, using broken-symmetry density functional theory (DFT) incorporated with the conductor-like screening (COSMO) solvation model. The calculated properties, including geometries, spin states, Fe-57, H-1, and O-17 hyperfine tensors, Mossbauer isomer shifts, and quadrupole splittings, and the estimation of the Fe(IV) d-d transition energies have been compared with the available experimental values. On the basis of the detailed analysis and comparisons, we propose a definite form for the active site structure of class I RNR intermediate X, which contains an Fe1(III)Fe2(IV) center (where Fe1 is the iron site closer to Tyr122, and the two iron sites are high-spin antiferromagnetically coupled to give a total (1)/(2) net spin), two mu-oxo bridges, one terminal water which binds to Fe1(III) and also H-bonds to both side chains of Asp84 and Glu238, and one bidentate carboxylate group from the side chain of Glu115.