Multistate Density Functional Theory of Excited States.

We report a rigorous formulation of density functional theory for excited states, providing a theoretical foundation for a multistate density functional theory. We prove the existence of a Hamiltonian matrix functional of the multistate matrix density () in the subspace spanned by the lowest eigenstates. Here, () is an -dimensional matrix of state densities and transition densities. Then, a variational principle of the multistate subspace energy is established, whose minimization yields both the energies and densities of the individual eigenstates. Furthermore, we prove that the -dimensional matrix density () can be sufficiently represented by nonorthogonal Slater determinants, based on which an interacting active space is introduced for practical calculations. This work establishes that the ground and excited states can be treated on an equal footing in density functional theory.