A v0-representability issue in lattice ensemble-DFT and its signature in lattice TDDFT
We study a small Anderson-impurity cluster using lattice density functional methods, and try todetermine the exact exchange-correlation (XC) potential via reverse engineering. In doing so we nd singlet{triplet degenerate interacting ground states which cannot be v0-represented in an ensemble-DFT sense. Wealso nd that it is possible to represent a triplet ground state as a pure state, but not the s