## 2 Simulation Methodology  
In this study, the mixed hydrogen-/propane- clathrate was stimulated by pairwise potentials; TIP4P/2005 type models for water \"which have been used in many simulation studies on hydrates\" [13]. Using TIP4P/2005, [PERSON] et al. showed an accurate sequence of energies in addition to structural parameters for ice polymorphs, in agreement with dispersion-corrected Density Functional Theory (DFT) [14]. In our previous recent studies [15; 16; 17], we showed the melting point of propane and propane/hydrogen hydrate can be predicted close to their experimental values using TIP4P/2005 potential. the General Amber Force Field (GAFF) used to handle Propane [18] and restrained electrostatic potential atomic partial charges (RESP) were sampled for propane from the HF/6-31G(d) electrostatic potential [19]. The model proposed by [PERSON] et al. was used to parametrise the hydrogen molecules [20]. Smooth particle-mesh Ewald method [21] was applied for long-range electrostatic and the temperature was controlled using Nose-Hoover-thermostat [22]. Initial coordinates of oxygen atoms in sII-clathrate unit cells were constructed from X-ray diffraction data [23]. The unit-cell lengths were 17.32 A; initial orientations of water molecules were chosen by the Rahman-Stillinger procedure [24] to comply with Bernal-Fowler rules [25] and have a negligible total-system dipole moment.  
The simulations were done for both uptake and release of hydrogen into/from propane hydrate. A \(2\times 2\times 2\) supercell was constructed and was put in contact in the \(+\) and \(-x\)-directions with the length of \(\sim\)129.75 each direction by vacuum. For the release simulation, every large cage was occupied by one propane molecule, while one hydrogen molecule located in every small one. The vacuum part is large enough to not avoid any significant pressure built up during the time of the hydrogen release process. In the case of uptake study, the same \(2\times 2\times 2\) supercell loaded only with propane molecules in large cages and keeping the small cages empty was sandwiched between relaxed hydrogen molecules with the length of 36 A along the \(\pm\) X-axis (i.e., a total hydrogen-phase length of 72 A). Assuming the hydrogen molecules as idea gas, the added hydrogen molecules can provide a pressure around 1000 bar at 273 K in the gas area. To afford compatibility with experimental data (in which case the cages at the interface are, ipso facto, complete before any thermal-imposed decomposition for heating above the melting temperature), extra water molecules were added to have complete cages at the interface [15]; cf. Figure 1.  
Before starting the production NVT simulations, to let surrounding gas-phase hydrogen molecules fully cover the clathrate surfaces, an initial MD runs at 280 K for 200 ps, with applied position restraint on the water molecules of the hydrate. Subsequent NVT production simulations, at various temperatures (180-273 K) were performed over 500 ns for release and 2 us for uptake study. [PERSON] and [PERSON] The clathrate-liquid-ice recognition metrics were employed to distinguish geometrically between the encapsulated vs. free gas molecules [25]. The gas molecules with minimum 8 hydrate like water molecules categorized as encapsulated gas inside clathrate hydrate cage.