Structure and energetics of molecular point defects in ice Ih

We present a first-principles study of the molecular vacancy and three distinct molecular interstitial structures in ice I-h. 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 I-h. A comparison between the vacancy and the Bc interstitial suggests that the former is the predominant molecular point defect for T less than or similar to 200K although a crossover scenario in which the latter becomes favored below the melting point is conceivable.