Transition path sampling of clathrate hydrate formation

Mixtures of methane gas & water forms ice-like solid methane hydrates (MH) via homogeneous nucleation. MH have important industrial and climate implications, yet their formation is poorly understood. Obtaining such understanding could lead to improved control of crystallization, as well as insight in polymorph selection in general, but is hampered by limited experimental resolution. Until now direct molecular dynamics simulations was used which could provide such insight, but is not feasible for realistic conditions due to rare event nature of nucleation. We harvest ensemble of the rare unbiased nucleation trajectories by employing Transition Path Sampling (TPS) and Transition Interface Sampling (TIS). These are trajectories that sample the free energy barrier across the liquid & solid phase. With an exorbitant simulation time of 4ms, we show that with decreasing undercooling the mechanism shifts from amorphous to crystalline polymorph formation. At intermediate temperature two mechanisms compete with each other. Reaction coordinate analysis reveals the amount of specific cage type is crucial for crystallisation, while irrelevant for amorphous solids. Polymorph selection is thus governed by kinetic accessibility of the correct cage type and occurs at precritical nucleus sizes against Ostwald’s step rule. We estimate the rate and nucleation barrier height for this process. Beyond this, we have explored carbon dioxide hydrate formation using similar methodology and got some novel and exciting new results. In the last part of the thesis, we applied machine learning algorithm to the data generated from TPS simulation of methane hydrate to find the reaction coordinate in the nucleation process. Overall, this thesis illustrates how a complicated process of nucleation can be fathomed crystal clearly, using path sampling.