Figure 1. Lowest-energy structures of Mg_mH_n(n ≥ 2m) clusters calculated at M062X/def2TZVP level. The green and white balls represent Mg and H atoms, respectively.

3.2 Hydrogen Adsorption of Mg_mH_nClusters

The adsorption energies (E_a) of Mg_mH_n (m = 3-6, n = 1-13) clusters are shown in Figure 2a. The adsorption energy of these structures was further verified with CCSD (T)/def2TZVP and shown in Table S1 in Supporting Information. Thermodynamically, the lower is the adsorption energy, the more stable the structure is. For Mg_mH_n (n < 2m), as the number of adsorbed hydrogen atoms increases, the adsorption energies of the clusters tend to be more negative until Mg:H = 1:2. The clusters with the stoichiometric ratio of Mg:H of 1:2 are the most stable. When the hydrogen atoms are further adsorbed, the adsorption energy of oversaturated Mg_mH_n (n = 2m+1) structures rises significantly, which indicates that Mg_mH_n (n = 2m+1) are less stable than saturated Mg_mH_n (n = 2m). However, it is noted that the hydrogen adsorption reaction in reality usually occurs under the high-pressure condition [38, 39], in which the hydrogen-enriched clusters can exist. As shown in Figure 2b, as the size of Mg_m clusters increases, the adsorption energy of Mg_mH_n (n = 2m+1) gradually decreases and the structure becomes relatively more stable. The adsorption energies of Mg₃H₇, Mg₄H₉ are all above zero, while for Mg₅H₁₁, Mg₆H₁₃, the adsorption energies are negative. These indicate that the formation of Mg₃H₇, Mg₄H₉ is endothermic reaction while the formation of Mg₅H₁₁, Mg₆H₁₃ is exothermic reaction.