However, the size of the periodic supercell created is usually not adequately large to avoid the unphysical electrostatic interaction between the defects and their periodic images. Such unphysical interaction will lead to the formation energies of the defect to include large convergence errors up to several eV magnitude. For reference, a difference of 0.65 eV (ca. 15 kcal mol -1-) may correspond to a reaction occurring at room temperature under standard atmospheric conditions. In charged defect states, the main source of errors arises from the spurious long-range Coulomb interaction between charged species and background charges. The latter is essential for calculating charged defects under the periodic boundary conditions. The most straightforward approach to resolve the spurious electrostatic interaction is by correcting a posteriori the predicted formation energies.