The adiabatic ground and excited states as well as all intermediate states for energy analysis are obtained from NOSI in multistate density functional theory. NOSI differs from nonorthogonal CI (NOCI) in that dynamic correlation is included in the basis states in the first place. Lu and Gao proved that the Hamiltonian in the subspace spanned by the lowest N eigenstates is a matrix functional 𝓗𝓗[𝑫𝑫(𝒓𝒓)] of the multistate density 𝑫𝑫(𝒓𝒓), extending Hohenberg-Kohn theorems for the ground state to any number of N states in MSDFT. Significantly, it was proven that 𝑫𝑫(𝒓𝒓) can be represented by no more than 𝑁𝑁 2 determinants, defining an upper bound for the number of determinants in an active space, i.e., a minimal active space (MAS), in excited-state DFT calculations. 𝑫𝑫(𝒓𝒓) can be variationally optimized by minimizing the trace of 𝓗𝓗[𝑫𝑫(𝒓𝒓)]. Then, Diagonalization of 𝓗𝓗 yields the exact energies for all 𝑁𝑁 adiabatic ground and excited states, provided that the exact correlation matrix functional is known (the Lu-Gao theorems established its existence). As in Kohn-Sham DFT, approximate correlation matrix functionals must be used in MSDFT calculations. The accuracy of the present MSDFT-NOSI method can be validated by comparison with results from TDDFT on systems that it is adequate as in the present case, employing the same correlation density functional. We focus on the lowest excited states of a monomer species and their interactions in the excimer complex. For the monomers and block-localized excitations in dimers in this work, we found that it is sufficient to just include 5 spin-adapted configurations in the MAS, consisting of the reference (ground) state from KS-DFT and four singly excited configurations from the two highest occupied KS orbitals (HOMO) to the two lowest unoccupied KS orbitals (LUMO). Each of the singly excited KS-determinants is optimized using the BLE technique, a form of ΔSCF method capable of block-localized constraints on the orbitals in the dimer case. Before presenting the MS-EDA results, we outline the procedure for determining the elements of the Hamiltonian matrix functional 𝓗𝓗[𝑫𝑫(𝒓𝒓)]. In particular, the diagonal elements )] are given by the KS-DFT energies of the corresponding determinants for the blocklocalized, excited configurations as well as that of the Kohn-Sham ground state.