In lithium fluoride (LiF), a self-trapped exciton (STE) forms when a fluoride ion (${\mathrm{F}}^{-}$) leaves its lattice site; the resulting anion vacancy traps the excited electron, becoming an $F$-center, while the displaced ${\mathrm{F}}^{-}$ moves to a nearby interstitial position, pairs with a neighboring fluoride ion to create an ${\mathrm{F}}_2^{-}$ molecular dimer, and localizes the hole as an $H$-center. Because the vacancy and the interstitial dimer sit next to each other, the self-trapped electron and hole remain nearest-neighbor partners. Here we present a comprehensive ab initio study of the structure and dynamics of STEs in LiF, focusing on their formation, stability, and dissociation processes. Using density functional theory with hybrid functionals and a supercell approach, we confirm that the $H$-center within the STE preferentially aligns along the $\langle 111\rangle$ direction, as determined via the Delta self-consistent field method. By employing the nudged elastic band method, we calculate migration barriers and reveal that the hopping barrier for the $H$-center remains nearly constant across different scenarios, including free $H$-center diffusion, the $H$-center orbiting around the $F$-center, and $H$-center separation during STE dissociation. This consistency stems from the weak interaction between the $F$ and $H$-centers in the STE, as indicated by the small energy difference between the STE configuration and the configuration where the $H$-center and the $F$-center are infinitely separated ($\mathrm{\Delta }{E}_{\text{STE-Free}}=0.064$ eV), suggesting that these centers behave almost as free entities after dissociation. Our results demonstrate that STE dissociation is energetically competitive with orbiting events at room temperature, leading to long-lived free electron-hole pairs. These findings provide crucial insights into exciton behavior in LiF, shedding light on the interplay between electronic localization and lattice distortions, which govern its optical and radiation response properties.

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• Color centers
• Diffusion
• Excitons
• First-principles calculations
• Polarons