Local density approximations and gradient-corrected functionals cannot describe bound
excitonic states due to lack of an effective attractive Coulomb interaction between photoexcited
electrons and holes. In contrast, hybrid density functionals, which include long-range nonlocal and
nonadiabatic corrections in a form of a fraction of Hartree-Fock exchange, are able to reproduce the
excitonic effects. The resulting finite exciton sizes are strongly dependent on the amount of the
orbital exchange included in the functional.