Kinetic energy as a density functional

The existence of a ground-state density functional for the kinetic energy of an N-electron system implied the existence of a density-functional derivative equivalent to a local potential function. No conflict was found between standard variational theory in the orbital-functional derivation and formal results of functional analysis. The Hohenberg-Kohn theorems ensured that variational energy was minimized by the ground-state density associated with a given external potential. The results showed that a density-functional derivative does exist for more than two electrons but has the mathematical character of a linear operator that acts on orbital wave functions.