  Improvement in the optoelectronic performance of halide perovskite
semiconductors requires the identification and suppression of non-radiative
carrier trapping processes. The iodine interstitial has been established as a
deep level defect, and implicated as an active recombination centre. We analyse
the quantum mechanics of carrier trapping. Fast and irreversible electron
capture by the neutral iodine interstitial is found. The effective Huang-Rhys
factor exceeds 300, indicative of the strong electron-phonon coupling that is
possible in soft semiconductors. The accepting phonon mode has a frequency of
53 cm$^{-1}$ and has an associated electron capture coefficient of
10$^{-10}$cm$^3$s$^{-1}$. The inverse participation ratio is used to quantify
the localisation of phonon modes associated with the transition. We infer that
suppression of octahedral rotations is an important factor to enhance defect
tolerance.
