In the framework of time-dependent density functional theory (TD-DFT), the first optical transition can usually be attributed to an excitation of a single electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The energy difference between the HOMO (EH) and the LUMO (EL) can be taken as a measure of the fundamental gap (Egap), which is more rigorously defined as the difference between the ionization potential and the electron affinity. At the equilibrium geometry Qeq = 0, the first vertical excitation energy (ES1; also called the optical gap) will differ from the fundamental gap (Eg) by the exciton binding energy (Ebind), which is a measure of the Coulombic attraction between the newly formed electron-hole pair: