Bismuth dopants have attracted intensive studies experimentally for their extremely broad near-infrared luminescence. Here we performed first-principles calculations to investigate the site occupancy and valence state by taking the condition of synthesis into consideration, and then calculated the excited states and various transitions of the bismuth ions by focusing on the targeted valent state Bi⁺ in a variety of ternary chloride MXCl₃ (M = K, Rb, Cs; X = Mg, Cd) hosts. The results on formation energies and charge transition levels show that vacant defects play an important role in the charge compensation for the bismuth dopants, and a lower chemical potential of chlorine benefits the stabilization of Bi+ at monovalent M sites. The multi-configurational quantum-chemical method and the constrained occupancy approach together confirm the near-infrared photoluminescence of Bi⁺, and the spontaneous emission rates due to electric–dipole and magnetic–dipole contributions are evaluated and analyzed in terms of transition selection rules, to affirm the Bi⁺ nature of the long lifetime luminescence. Our results show that the mechanisms revealed in this study, and the combination of density-functional calculations for defect formation energies with the wave-function based calculations for optical transitions, are effective in exploring the luminescence of bismuth dopants in solids. Copyright © 2021 the Owner Societies.