Table lists the energies of the ground state and the 𝑛𝑛 → 𝜋𝜋 excited state from MSDFT-NOSI and TDDFT calculations relative to that from KS-DFT for acetone. The results for the 𝐿𝐿 𝑎𝑎 (𝑆𝑆 1 ) and 𝐿𝐿 𝑏𝑏 (𝑆𝑆 2 ) states of pentacene are given in Table . The performance of NOSI and possible double counting of electron correlation in a multistate DFT can be validated by comparison of the ground state energies with that from KS-DFT calculations. We find that the difference in ground-state energy between MSDFT and KS-DFT is generally small, with the largest deviation being just -0.02 eV for acetone. Note that the KS-DFT state is included in the active space in all NOSI calculations. This observation indicates that there is little double-counting of correlation in NOSI for the present systems using the M06-2X functional. Importantly, the inclusion of the Kohn-Sham ground-state ensures orthogonality of the computed excited states to the ground state. Listed in Tables and are relative energies for the ground and excited states from NOSI and TDDFT calculations employing monomer geometries optimized for 𝑆𝑆 0 and 𝑆𝑆 1 states and monomers in the optimized monomer structure. In addition, we included the computed excitation energies of the excimer complexes. The agreement between NOSI and the linearresponse theory is good with a root-mean-square deviation (RMSD) of 0.22 eV, which is largely due to the somewhat greater differences between NOSI and TDDFT for the 𝐿𝐿 𝑏𝑏 state of pentacene using M06-2X. If only 𝐿𝐿 𝑎𝑎 states of pentacene are included the RMSD error is less than 0.01 eV.