As discussed above, there is a counterpart for each lowest-energy structure of AlBe_n in the minimum structures of their corresponding Be_{n +1} cluster. Besides, as can be seen in Table 1, the number of Al–Be bonds in AlBe_n increases with the increasing cluster size and reaches a maximum value of six. It implies that the impurity Al atom is able to bond with six Be atoms at most. These rules may help to identify the low-lying structures of larger Al-doped Be_n clusters.

The comparison between AlBe_n and other Al-doped clusters shows how the relative size of impurity atom affects the structural evolution of the whole system. For example, the impurity Al atom always occupies an external position of the host clusters in AlBe_n and AlB_n ^[40] until n = 12, while it gets trapped in the host cage from n = 6 onwards in AlLi_n ^[41] and AlNa_n ,^[42,43] and fromn = 9 onwards in the AlTi_n ,^[44]AlSc_n ,^[45]AlPb_n ,^[46] and AlY_n ^[47] clusters. These structural distinctions can be related to the size difference of the dopant atom versus the host atom. To be specific, the impurity Al atom (1.25 Å)^[48] has larger atomic radius than those of B (0.85 Å) and Be (1.05 Å) atoms, which hinders it from entering the B_n or Be_n cages. In contrast, a framework constituted by larger host atoms such as Li (1.45 Å), Na (1.80 Å), Ti (1.40 Å), Sc (1.60 Å), Pb (1.80) and Y (1.80 Å) can accommodate the dopant Al atom.