The DFT/CIS formalism is very similar to CIS, with a few important modifications necessitated by the use of Kohn-Sham orbitals. Our notation is that ψ i , ψ j , . . . denote occupied MOs and ψ a , ψ b , . . . are virtual MOs, with indices r, s, . . . referring to arbitrary MOs. The quantity |Ψ a i represents a singly-substituted Slater determinant constructed from a reference determinant |Ψ 0 that is the solution to the self-consistent field (SCF) problem. Using a closed-shell formalism for simplicity (although our implementation includes the spin-unrestricted case), the matrix elements of the CIS Hamiltonian are 9