CaH2 hydrogenation occurs via surface melting at high temperatures. Figure shows snapshots of a MLP-MD simulation of the CaH2(100) surface in contact with high-density H2 at P = 40 GPa and T = 1500 K. During MLP-MD simulation, the surface structure was disordered (ex. t = 100 ps and 116 ps) to absorb H atoms into the bulk. Since CaH2 lacks H2molecule-like dimers (H2 δ-), whereas CaH4 contains them, confirming H absorption during disordering from the purple lines in the bulk. After this H absorption, an ordered structure including H2 δ-was formed, i.e. solid CaH4 (t = 160 ps.). Figure The black line in Fig. shows the results of MLP-MD simulation at T = 1200 K. During the MD simulation, the H/M ratio slightly increased due to the H2 adsorption on the surface and H2 absorption in the subsurface layer. However, the H/M ratio was still close to two, showing that the surface melting and subsequent hydrogenation did not occur at T = 1200 K. Figure (b) shows the adaptive common neighbor analysis, a-CNA , of Ca atoms during MLP-MD simulation. During the structural disordering, Ca formed neither an hcp Ca nor an fcc Ca lattices. This shows that CaHx during hydrogenation formed an amorphous or liquid-like structure rather than solid CaH2 or CaH4. After 120-ps, the fcc Ca ratio (≈ CaH4) increased, confirming the formation of solid CaH4. Note that the fcc Ca ratio was considered to be underestimated due to the large thermal oscillation at 1500 K. In fact, when this calculation cell was quenched to 300 K, the fcc Ca ratio increased to 86 %.