Beryllium, Beryllium Clusters, and Beryllium-Containing Molecules. There has been a plethora of studies on beryllium, beryllium clusters, and beryllium-containing molecules, and they can be used in a wide variety of applications.2-24 An important motivation for studying beryllium is that its clusters serve as a prototypical series that can be used to answer questions that are essential to understanding metal clusters and their bonding motifs. These questions include, at what size does the cluster begin to obtain characteristics of bulk metal, and how does bonding evolution affect the properties of growing clusters? These questions are especially challenging as the answers depend upon properties that are often difficult to measure, such as electron affinity (EA), geometric structure, bond energies, ionization energy, and the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Beryllium, due to its small atomic radius and high ionization potential (9.32 eV), exhibits unique chemistry with a large degree of covalency when compared to other metals in Group IIA.10 The small np-ns energy intervals also contribute to the covalent tendencies of beryllium, and the evolution of covalency is theoretically predicted as the size of a pure beryllium cluster increases. The degree of hybridization increases, and the bond energy substantially increases when the beryllium cluster grows from the dimer to the cyclic trimer. A large increase in the s-p hybridization is also observed. However, other physical properties of the clusters evolve non-monotonically. It has been demonstrated that a nonrelativistic Hamiltonian is sufficient for studying the Be atom, which indicates that, when paired with large basis sets, high-level computational treatments are plausible methods for studying beryllium clusters. Experimental and theoretical studies have shown that Be2 has a weak bond with a formal bond order of zero while bulk beryllium exhibits a high melting point and is a hard metal.11 These findings also indicate nonmonotonic cluster evolution, where many-body nonadditive forces affect the cluster energies. Electronic structure calculations have predicted that the ionization energy and HOMO-LUMO band gap of Be clusters oscillate in a pattern that is indicative of an electronic shell structure and could be due to the close relationship between the electronic and geometric structure of the clusters.25-38 Understanding beryllium clusters and determining their properties can shed light on...
