Excimers play an important role in photochemical processes ranging from singlet fission to DNA damage, and the characteristic red-shift in fluorescence spectra associated with excimer formation can provide information about aggregate formation and the orientation of chromophores. When a mixture of chromophores is present, exciplex formation may lead to spectral characteristics distinct from those of either monomer or the corresponding excimers. To predict the effects of aggregation in a system containing a mixture of small acenes, binding energies and minimum-energy geometries have been calculated for three mixed S_1 exciplexes. Benchmark CASSCF/NEVPT2 multireference binding energies of 18.2, 27.7, and 49.3 kJ/mol are reported for the benzene-naphthalene, benzene-anthracene, and naphthalene-anthracene exciplexes, respectively. TDDFT calculations have been performed using a range of exchange-correlation functionals, showing that many functionals perform inconsistently, and the error in binding energy often depends on the character of the monomer excitation from which the exciplex state is derived. Moderate exciplex stabilization observed for the benzene-naphthalene and naphthalene-anthracene exciplexes results from a mixture of charge transfer and exciton delocalization
