As SIESTA code employs NAOs, the program replaces some integrals in real space by sums in a finite three dimensional real space grid, using one single parameter to control the PD173074 cutoff of the grid , which refers to the fineness of the grid, and was converged for all the systems studied here, finding an appropriate value of 250 Ry with an energy shift of 0.01 eV. To sample the Brillouin zone, Monkhorst–Pack method  was used employing k-point meshes of 4 × 4 ×1 for the slab system. The supercell employed was tetragonal with x, y, z periodic boundary conditions. The Fe(1 0 0) surface was emulated by a p(2 × 2) 5-layer slab, with three bottom layers fixed and two top layers free to relax. To model the solid/vacuum interphase, a large unit cell vector in z direction was selected, so that the interaction between the system and their images became negligible. The vacuum region was 1.5 nm thick. The determination of the minimum energy path for the different reactions was undertaken using the Nudged Elastic Band (NEB) method  and  and the local minima were found through the conjugate gradient (CG) technique.