Metallic and molecular orbital concepts in XMg8 clusters, X = Be-F.

The electronic structure and stability of the XMg(8) clusters (X = Be, B, C, N, O, and F) are studied using first principles theoretical calculations to understand the variation in bonding in heteroatomic clusters which mix simple divalent metals with main group dopants. We examine these progressions with two competing models, the first is a distorted nearly free electron gas model and the second is a molecular orbital picture examining the orbital overlap between the dopant and the cluster. OMg(8) is found to be the most energetically stable cluster due to strong bonding of O with the Mg(8) cluster. BeMg(8) has the largest HOMO-LUMO gap due to strong hybridization between the Mg(8) and the Be dopant states that form a delocalized pool of 18 valence electrons with a closed electronic shell due to crystal field effects. Be, B, and C are best described by the nearly free electron gas model, while N, O, and F are best described through molecular orbital concepts.