First-principles calculations of the self-trapped exciton in crystalline NaCl

The atomic and electronic structure of the lowest triplet state of the off- center (C-2 nu symmetry) self-trapped exciton in crystalline NaCl is calcul ated using the local-spin-density (LSDA) approximation. In addition, the Fr anck-Condon broadening of the luminescence peak and the a(1g) --> b(3u) abs orption peak are calculated and compared to experiment. LSDA accurately pre dicts transition energies if the initial and final states are both localize d or delocalized, but I eV discrepancies with experiment occur if one state is localized and the other is delocalized.