The electronic structure of indigo was determined using B3LYP functional and different basis sets. The first triplet (3)A(u) (C-2h) state of indigo is characterized by the singlet-triplet gaps of Delta ET - S(vertical) = 1.23 eV and Delta ET - S(adiabatic) = 0.95 eV (recent experiment: 1.04 +/- 0.1 eV). The vertical S-1 - T-1 gap amounts to 1.0 eV (exptl: 0.91 +/- 0.1 eV). The electron localization function (ELF) and spin density analysis show that the singlet-triplet excitation is accompanied by a migration of electrons from the C=C bond and N-atoms to the adjacent C-C and C-N bonds. A low ionization energy is confirmed for indigo IEa = 6.9 eV. (c) 2007 Elsevier B.V. All rights reserved.