The local structure around H atoms during surface melting is similar to that around H atoms in CaH4, providing an increased number of absorption sites compared with local environments for H atoms in CaH2. Figure shows a time-averaged persistence diagram (PD) of the MLP-MD simulation of the CaH2(100)/H2 interface at 1500 K under 40 GPa with surface melting (rainbow color density plot) compared with that at 1200 K under 40 GPa without surface melting (grayscale contour plot). In Fig. , new ring structures appear around (birth, death) = (b, d) = (1.0, 2.1) only when CaH2 hydrogenation occurs. This shows that these rings are important for the CaH2 hydrogenation reaction. Figure (a) shows the pressure-dependence of the H/M ratio after 1-ns MLP-MD simulations. Here, (100) and (010) surfaces were analyzed. From Fig. (a), it is confirmed that the CaH2 hydrogenation reaction occurs at 1500 K above 30 GPa in this MLP. Conversely, hydrogenation does not occur at 1200 K at any pressure. This indicates that surface melting is a thermally activated process, whose reaction temperature is between 1200 K and 1500 K. At the CaH2(010)/H2 interface, hydrogenation does not occur at any pressure (blue circles). This result is associated with the surface energies of (100) and (010). The surface energy of (010) in vacuum is determined to be 0.034 eV/Å by DFT calculation, which is lower than 0.04 eV/Å of (100). Thus, although this is a qualitative discussion based on the surface energy under vacuum conditions, this hydrogenation behavior suggests that the surface melting is affected by the interfacial energy between CaH2 and H2. Another possible reason for hydrogenation occurring only on the (100) surface is the difference in the in-plane density of Ca atoms between the (100) and ( ) surfaces. The CaH₂(010) surface forms a layer of close-packed hcp Ca atoms as shown in the inset in Fig. . In contrast, the CaH₂(100) surface exhibits a stepped structure (the inset in Fig. (a)), with a lower atomic density in the same plane compared to the (010) surface.