Different applications impose different requirements on the excited-state energies of molecular crystals. For example, solar cells require materials with broad absorption in the visible range, whereas OLEDs require materials with sharp emission in specific colors. Other applications may require absorption in the infrared (IR) or ultra-violet (UV) ranges. SF requires the triplet excitation energy to be slightly smaller than half the singlet excitation energy, whereas TTA requires the triplet energy to be slightly larger than half the singlet energy. TADF requires the difference between the singlet excitation energy and the triplet excitation energy, also known as the singlet-triplet gap, to be as small as possible or even negative. In addition, compatibility with other device components may require singlet and/or triplet energies in a specific range and/or specific band edge positions. Although the structures of over a million organic molecular crystals are known, the electronic and optical properties of most of them are unknown. A material originally produced for one purpose may turn out to be useful for another. Even materials that formed as reaction byproducts may turn out to have useful properties. Therefore, the ability to predict the excited-state properties of molecular crystals could lead to materials discovery and advances in organic semiconductor devices.