Experimental absorption and fluorescence energies for acetone are 4.5 and 2.8 eV in the gas phase, and a acetone excimer fluorescence energy of 3.06 eV has been reported; For MS-EDA, we first consider the asymmetric acetone excimer, which was optimized for the 𝑆𝑆 1 state of the complex using TDDFT with the M06-2X functional. We obtained a structure that is best characterized as the local excitation of one monomer that is distorted at the carbonyl carbon from sp 2 hybridization (Figure ), forming a tail-to-tail dimer with an in-phase combination of the distorted 𝜋𝜋 * orbital of the two acetone molecules as the highest singly occupied orbital. The vastly different monomer structures here turn out to be equivalent to a general exciplex complex if the two compounds were different. Interestingly, the distorted acetone monomer in the excimer complex, (𝐴𝐴𝐴𝐴) 𝑆𝑆 1 , is somewhat less deformed by about 0.1 eV (2.4 kcal/mol) than the optimized geometry, (𝐴𝐴) 𝑆𝑆 1 , in isolation, but the adiabatic excitation energies (3.8 eV) are essentially the same (Table ). The agreement between NOSI and TDDFT calculations is good for the four geometries considered (Table ).