Positron annihilation lifetimes from F-center belated bound states of alkali halides

First-principle density functional calculations of positron annihilation lifetimes have been carried out for the positron bound F-center defects in alkali halides. The system is modeled as an electron with basis functions only on the halide vacancy in a simplified cluster-embedding scheme. The rest of the crystal is modeled as 1) point-ions located on the lattice positions and 2) the frozen-orbital ions derived from an energy band calculation of the pure crystal. Both calculations are carried out within the self-interaction corrected local spin density approximation and incorporate an electron-positron correlation functional. We analyze the effect of the model assumptions on calculated positron lifetimes and demonstrate the sensitivity of the results to the inclusion of the crystal field. Our results show that the electron-positron correlation is the most significant ingredient on the positron annihilation on the F-center and the agreement from the comparison to other empirical calculations and the experimental data is remarkable.