Extension of the absorption properties of TiO2 photocatalytic materials to the visible part of the solar spectrum is of major importance for energy and cleaning up applications. We carry out a systematic study of the N-doped anatase TiO2 material using spin-polarized density functional theory (DFT) and the range-separated hybrid HSE06 functional. The thermodynamic stability of competitive N-doped TiO2 structural configurations is studied as a function of the oxygen chemical potential and of various chemical doping agents: N2, (N2 + H2), NH3, N2H4. We show that the diamagnetic TiO (2-3x)N2x system corresponding to a separated substitutional N species (with 2-4% N impurities) and formation of one-half concentration of O vacancies (1-2 atom %) is an optimal configuration thermodynamically favored by NH3, N2H4, and (N2 + H2) chemical doping agents presenting a dual nitrating-reducing character. The simulated UV-vis absorption spectra using the perturbation theory (DFPT) approach demonstrates unambiguously that the diamagnetic TiO(2-3x)N2x system exhibits the enhanced optical absorption in N-doped TiO2 under visible-light irradiation. Electronic analysis further reveals a band gap narrowing of 0.6 eV induced by delocalized impurity states located at the top of the valence band of TiO 2. A fruitful comparison with experimental data is furnished. © 2011 American Chemical Society.