Comment on “Kinetic energy as a density functional”

In a recent paper, Nesbet [Phys. Rev. A 65, 010502(R) (2001)] has proposed dropping “the widespread but unjustified assumption that the existence of a ground-state density functional for the kinetic energy, T_s[ρ], of an N-electron system implies the existence of a density-functional derivative, δT_s[ρ]/δρ(r), equivalent to a local potential function,” because, according to his arguments, this derivative “has the mathematical character of a linear operator that acts on orbital wave functions.” Our Comment demonstrates that the statement called by Nesbet an “unjustified assumption” happens, in fact, to be a rigorously proven theorem. Therefore, his previous conclusions stemming from his different view of this derivative, which undermined the foundations of density-functional theory, can be discounted.