The electron energy density is defined in Eq. ( ), as a product of the physically observable electron density 𝜌(𝒓) and the average electron energy 𝜒̅ (𝒓) . The latter quantity can be approximated from an average of occupied molecular orbitals. An important extension of our approach is to allow for the study of extended systems by expressing 𝛸(𝒓) not in terms of KS average orbitals, but as a sum of energy densities.