In Panel (a) single-molecule and crystal DFT quantities are compared to the GW+BSE crystal optical gap. The fundamental gap of a molecule corresponds to the difference between the ionization potential (IP) and electron affinity (EA). The fundamental gap of a molecular crystal (calculated by GW ) is typically significantly narrower than the single molecule fundamental gap because of screening and band dispersion in the crystal. The optical gap of a molecular crystal is narrower than the fundamental gap because of the exciton binding energy. The IP and EA calculated based on on DFT total energy differences Although there is correlation with the overall trend of the GW +BSE optical gaps, the spread of the PBE IP-EA values is too large to be considered as a reliable predictor. The single molecule PBE HOMO-LUMO gap is particularly attractive for this purpose because it is very fast to evaluate.