The local arrangement of a substitutional Cr3+ion for an Al3+ion in corundum is studied by means of first-principles pairwise simulations and quantum-mechanicalab initioPerturbed Ion calculations. Our investigation is organized in two steps. First, we determine the cohesive properties of the host lattice by calculating the set of four crystalline parameters that makes minimum the total energy of corundum. Secondly, we solve cluster models of increasing complexity centered at the Cr3+site and embedded in the previously computed crystal potential. This is a consistent strategy that contributes to determine the local geometry of Cr3+in α-Al2O3.