| Density Functional Theory has been successfully applied in solid state calculations where it has been customary to extract just a single number, namely the total energy. Within this formalism, we show that we can, in addition, compute an energy density which is useful despite its inherent non-uniqueness, and which comprises of a kinetic energy density, a Maxwell energy density which describes the long-range Coulomb interaction, a term that includes the exchange-correlation interaction, and other contributions that depend on the external potential.; We present explicit calculations of the energy density within the Local Density Approximation and we study selected polar (111) surfaces and interfaces. Our technique enables the calculation of the formation enthalpy of a single isolated polar (111) or {dollar}(overline{lcub}111{rcub}){dollar} surface or interface which is not possible using conventional total energy methods. We present surface energy results for the test Si(111) ideal surface to outline the method of calculation. We compute, for the first time, the formation enthalpies for the GaAs (111) and {dollar}(overline{lcub}111{rcub}){dollar} surfaces, and the Ga-rich and Al-rich interfaces in the GaAs/AlAs (111) heterojunction. Our results enable us to address questions about the stability of these structures. |
