Ferromagnetic bonding in high-spin alkali-metal clusters. How does sodium compare to lithium?

High-spin sodium clusters (ⁿ⁺¹Na_n; n = 2-12) are studied using a combined density functional (B3P86/cc-pVDZ) and UCCSD(T)/cc-pVDZ approach. In agreement with previous findings for high-spin lithium clusters, here too the minimum energy clusters are compact species with a high coordination number for each atom in the cluster. Generally, though, high-spin bonding in sodium clusters is much weaker than in the corresponding high-spin lithium clusters. Furthermore, the electronic configuration and the ordering of the molecular orbitals of high-spin sodium clusters differ from those in the high-spin lithium clusters. The electronic states of high-spin sodium clusters have much less p-character than corresponding high-spin lithium clusters. Molecular orbital and valence bond considerations show that the reduced p-participation in bonding is the root cause of the weak ferromagnetic bonding in sodium vs. lithium clusters.