(2.1.12) Yang : Indeed, the dual formulation of DFT is the potential-functional theory (PFT). PFT establishes two results: the dual of the Hohenberg-Kohn theorem in terms of the external potential as the basic variable and the dual of the Kohn-Sham theorem in terms of the potential of the noninteracting reference system. The first result provides a solution to the v-representability problem in the original Hohenberg-Kohn work. The second result provides the theoretical foundation for the optimized-effectivepotential approach for Kohn-Sham calculations with functionals of orbitals.