 From a purely macroscopic, thermodynamic perspective, Berthollides can be understood in terms of the minimization of the thermodynamic function called the Gibbs free energy, which accommodates the interplay of energy and entropy as functions of temperature and pressure. Stable substances are ones with minimal Gibbs free energy. On the microscopic scale, the basic microstructure of ferrous oxide is a three-dimensional lattice of ferrous (Fe2+) and oxide (O2-) ions. However, some of the ferrous ions are replaced by holes randomly distributed in the crystal lattice, which generates an increase in entropy compared with a uniform crystal structure. An overall imbalance of electrical charge would be created by the missing ions. But this is countered in ferrous oxide by twice that number of ions from those remaining being converted to ferric (Fe3+) ions. This removal of electrons requires an input of energy, which would make for a less stable structure were it not for the increased entropy afforded by the holes in the crystal structure. The optimal balance between these forces depends on the temperature and pressure, and this is described by the Gibbs free energy function. 