This allows for formulating the formation energy in dependence on ∆µ In and ∆µ Al , i.e., the variations of the In and Al chemical potentials with respect to their bulk values. For the heat of formation ∆H f,AlInP2 we calculate a value of -1.49 eV. The hydrogen chemical potential, ∆µ H with respect to an isolated molecule, provides an additional and independent degree of freedom. In the approximation of a two-atomic ideal gas, it is written in dependence on partial pressure p and temperature T as