In conventional Kohn-Sham DFT, the approximate energy functional Ẽ[ρ] is minimized with respect to ρ. The resulting energy approximates what would be obtained via optimization of an exact functional E[ρ], if the latter were known. Burke, Sim, and coworkers suggest that it is pedagogically useful to separate the error Ẽ -E into two contributions: a functional-driven part