The hydrogen storage capacity in the clathrate hydrate was studied by ab initio calculations and ab initio molecular dynamics simulations. Thermodynamic and kinetic analysis shows that the cage occupancy in small and large cages is affected by each other, and THF has a stabilization effect on the hydrate structure. For pure H-2 hydrates, small cages can be occupied by single H-2 molecule or double H-2 molecules, while the corresponding occupancy in large cages is four or three H-2 molecules, resulting in a hydrogen storage capacity of similar to 3.8 wt% and similar to 4.4 wt%, respectively. For binary H-2-THF hydrates, small cages are likely to be singly occupied with H-2, but large cages can simultaneously accommodate one H-2 molecule and one THF molecule. The hydrogen storage capacity falls in between similar to 1.6 wt% and similar to 3.8 wt%. This study highlights the importance of the clathrate hydrates as a hydrogen storage material and also is helpful to understand the controversy about the hydrogen storage capacity in the clathrate structure. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.