Despite the potential applications and ubiquity of clathrate hydrates, the molecular mechanism of formation of these compounds is not yet well-understood. In this work, the formation mechanism of the face-sharing double cages in structure-I hydrate was studied by density functional theory calculations, which is responsible to the hydrate nucleation and growth. The results show that the clathrate cages favor to form one after another, and the 512 cages are thermodynamically feasible in the beginning. The water-water and water-methane interactions mostly dominate the formation of the clathrate cages, while the methane-methane interactions have little effect on the formation process. (C) 2017 Elsevier B.V. All rights reserved.