The Mn Is near-edge absorption fine structure (NEXAFS) has been computed by means of transition-state gradient-corrected density functional theory (DFT) on four Mn4Ca clusters modeling the successive S-0 to S-3 steps of the oxygen-evolving complex (OEC) in photosystem II (PSII). The model clusters were obtained from a previous theoretical study where they were determined by energy minimization. They are composed of Mn(III) and Mn(IV) atoms, progressing from Mn(III)(3)Mn(IV) for S-0 to Mn(III)(2)Mn(IV)(2) for S-1 to Mn(III)Mn(IV)(3) for S-2 to Mn(IV)(4) for S-3, implying an Mn-centered oxidation during each step of the photosynthetic oxygen evolution. The DFT simulations of the Mn Is absorption edge reproduce the experimentally measured curves quite well. By the half-height method, the theoretical IPEs are shifted by 0.93 eV for the S-0 -> S-1 transition, by 1.43 eV for the S-1 -> S-2 transition, and by 0.63 eV for the S-2 -> S-3 transition. The inflection point energy (IPE) shifts depend strongly on the method used to determine them, and the most interesting result is that the present clusters reproduce the shift in the S-2 -> S-3 transition obtained by both the half-height and second-derivative methods, thus giving strong support to the previously suggested structures and assignments.