where {Φ I } are trial Kohn-Sham wave functions. While the above definition is formally convenient because it ensures that the Kohn-Sham orbitals are obtained from a single (local) ensembledensity-functional potential (Hartree-only in this case), it is, from a practical point of view, a very poor choice. The reason is that it contains unphysical "ghost" interaction terms between the states. The Hartree energy defined in this way also varies quadratically with the ensemble weights, by construction, while the exact ensemble energy varies linearly. At first sight, it seems better to opt for the following definition,