can be found from Figure S2 that, the Al charge increases as the size of cluster grows and tends to flatten out from AlBe9onwards. Note that this varying trend roughly corresponds to that of coordination number of Al in the AlBen clusters, suggesting that higher coordination number of impurity atom is beneficial to intracluster charge transfer in this case. According to Table 3, the 3s states of Al lose 0.04–0.31 electrons, while the 3p states get 0.18–2.48 electrons. As for the Be atoms, their 2s orbitals lose 0.39–0.93 electrons, while the 2p states get 0.22–0.76 electrons. Hence, the charge transfer within the AlBenclusters is mainly from the 3s orbital of Al and 2s orbitals of Be atoms to the 3p orbital of Al and 2p orbitals of Be atoms. By contrast, the contributions from other orbitals of Al and Be are negligible.
The HOMO-LUMO gaps of the AlBen clusters at the B3LYP level are listed in Table 1 and plotted in Figure S3. From the figure, there are three obvious peaks at AlBe3, AlBe5 and AlBe8, indicating relatively high stability of these three clusters. In particular, the AlBe8 cluster possesses the largest HOMO–LUMO gap of 2.817 eV, which not only is larger than the experimental gap value of 1.9 eV for the kinetically stable C60,[47] but also exceeds that of 2.53 eV (computed at the same level) for the chemically inert superatom Al13.[48] For comparison, the HOMO–LUMO gaps of the Ben +1clusters are also depicted in Figure S3. It can be found that substituting an Al atom for a Be atom enlarges the HOMO–LUMO gaps of Be8 and Be10 clusters by 0.689 eV and 0.398 eV, respectively, while the Al substitution effect is insignificant for Be11 and Be13 clusters and is negative for the other beryllium clusters.
Finally, the evolution of polarizability per atom of AlBen (n = 1–12) is considered since the static polarizability is an important measure of electronic properties of clusters. From the results shown in Figure 4, a turning point atn = 3 can be found, which squares with the geometry transformation from planar to steroescopic at AlBe3. For comparison, the polarizability per atom of the lowest-energy Ben +1 clusters are also given in Figure 4. From the figure, both curves show generally decreasing trends with increasing cluster size, and gradually become stable for larger sizes. Besides, substituting a Be atom in Ben +1 with an Al atom always brings about a larger polarizability to the doped cluster, although the Al-substitution effect is less prominent for larger-sized ones. Hence, the AlBen clusters are more polarizable compared with their corresponding Ben +1 clusters. Note that there is a relatively large polarizability gap between Be6 and AlBe5, which can be attributed to the large structural discrepancy between these two clusters since polarizability is sensitive to the shape of the system.[50]