One way to construct the multistate active space is to use constrained Kohn-Sham determinants for the states of interest. These non-Aufbau configurations can be optimized via a ∆SCF procedure, or by the targeted orbital optimization method. If we do not simultaneously optimize {c AI , a pA } in eqn ( ), the orbitals in each determinant will be separately optimized, and they are generally nonorthogonal. Then, the procedure is equivalent to nonorthogonal state interaction (NOSI), a convenient approximation to the full MSSCF solution. In this case, one first carries out the optimization of each determinant configuration as a constrained Kohn-Sham DFT calculation. Then, this is followed by a single step of diagonalization of the Hamiltonian matrix functional (eqn ( )), with the approximations for the off-diagonal elements H AB given in refs 454,455,457 to yield the energies of the adiabatic states. Consequently, all adiabatic states in the ensemble, including the ground state, are treated on an equal footing in the course of minimizing the ensemble density functional. Since state interactions are explicitly included in the active space that defines the ensemble energy, the effect of interference highlighted in contribution (3.7.1) is naturally included in MS-DFT.