Identification of Clathrate Hydrates, Hexagonal Ice, Cubic Ice, and Liquid Water in Simulations: the CHILL plus Algorithm

Clathrate hydrates and ice I are the most abundant crystals of water. The study of their nucleation, growth, and decomposition using molecular simulations requites an accurate and efficient algorithm that distinguishes water molecules that belong to each of these crystals and the liquid phase. Existing algorithms identify ice or dathrates, but not both. This poses a challenge for cases in Which ice and hydrate coexist, such as in the synthesis of dathrates from ice and the formation of ice from dathrates during self-preservation of methane hydrates. Here We present an efficient algorithm for the identification of dathrate hydrates, hexagonal ice, cubic ice, and liquid water in molecular simulations. CHILL+ uses the number of staggered and eclipsed water-water bonds to identify water molecules in cubic ice, hexagonal ice, and dathrate hydrate. CHILL+ is an extension of CHILL (Moore et al. Phys. Chem. Chem. Phys. 2010, 12, 4124-4134), which identifies hexagonal and cubic ice but not dathrates. In addition to the identification of hydrates, CHILL+ significantly improves the detection of hexagonal ice up to its melting point. We validate the use of CHILL+ for the identification of stacking faults in ice and the nucleation and growth of dathrate hydrates. To our knowledge, this is the first algorithm that allows for the simultaneous identification of ice and dathrate hydrates, and it does so in a way that is competitive with respect to existing methods used to identify any of these crystals.