We present a first-principles study of the molecular vacancy and three distinct molecular interstitial structures in ice Ih. The results indicate that, due to its bonding to the surrounding hydrogen-bond network, the bond-center (Bc) configuration is the favored molecular interstitial in ice Ih. A comparison between the vacancy and the Bc interstitial suggests that the former is the predominant molecular point defect for T approximately < 200K although a crossover scenario in which the latter becomes favored below the melting point is conceivable.
De Koning, M., Antonelli, A., Da Silva, A. J. R., & Fazzio, A. (2006). Structure and energetics of molecular point defects in ice Ih. Physical Review Letters, 97(15). https://doi.org/10.1103/PhysRevLett.97.155501