Here, |Φ ia is obtained from the ground state configuration by ψ iσ → ψ aσ excitation. Strictly speaking, LR-TDDFT affords only the transition density ρ 0K (r, r ) [Eq. ( )], not the excited-state wavefunction per se, but analogies along the lines of Eq. ( ) have been in used since the earliest days of the theory. In condensed-matter physics, it is also common to identify the product functions ψ a (r elec ) ψ * i (r hole ) as a quasiparticle basis for electron-hole pairs ("excitons"), and to consider ρ 0K (r elec , r hole ) to be an "exciton wavefunction". This is despite the fact that the Green's function methods that are often used in that context afford transition densities rather than proper wavefunctions, just like LR-TDDFT.