In this study, to provide a physical insight into the superhydride synthesis, we directly observed CaH2 hydrogenation by MLP-MD simulations. CaH2 is an ionic crystal composed of Ca 2+ and H -with an hcp Ca lattice . On the other hand, CaH4 forms an fcc Ca lattice , which contains H -ions and H2 δ-dimers. Therefore, the CaH2 hydrogenation reaction, CaH2 + H2 ↔ CaH4, is employed as an example of superhydride synthesis that requires (1) metal lattice reconstruction and (2) hydrogenation starting from stoichiometric ionic crystals whose anion sites are fully occupied by H -.