Time-dependent density functional theory (TD-DFT), the most widely used quantum chemistry technique for computing electronic excitation energies, has a simple and conceptually pleasing one-electron interpretation in terms of a particle (the excited electron) and a hole. Excited-state properties can be decomposed into contributions from each of these quasiparticles. In particular, the particle and hole densities can be used to define a mean electron-hole distance (d e-h ) in the excited state, d e-h = r elec -r hole .