The stability orders of a number of alkaline earth oxide cluster isomers , M = Mg, Ca, Sr, Ba and have been determined by means of density functional theory studies using the LDA-PWC functional. Among the candidate structures, the hexagonal-ring-based isomers and the slab shapes are found to display similar stabilities. Stacks of hexagonal (MO)3 rings are found to be the slightly preferred growth strategy among the (MgO)6, isomers. In contrast, the slab structures are slightly preferred for the other alkaline metal oxide (MO)6 clusters. An explanation based on packing and aromaticity arguments has been proposed. This study may have important implications for modeling and understanding the initial growth patterns of small nanostructures of alkaline earth metals. 1. Introduction In the last few years, considerable effort has been directed to the understanding of metallic and semiconductor clusters. Clusters are aggregates of atoms or molecules intermediate in size between individual atoms and bulk matter, and their studies provide an interesting way to develop materials with varying properties by changing size and shape. Hence, studies of cluster properties as a function of size have received prominence in recent years. While much progress has been made on clusters of metals and semiconductors, metal oxide particles are often considered to be bulk fragments. However, their structure and properties could be entirely different in small clusters [1–4]. In this work, we have performed a comparative study of the structures, stabilities, and properties of some alkaline earth metal oxides ( , , , and ). Magnesium oxide crystallizes in the rock-salt structure and has some typical semiconducting properties, such as wide valence band (~6？eV), large dielectric constant (9.8), and small exciton binding energy (<0.1？eV). For bulk MgO, the experimental value of the band gap is 7.8？eV [5]. It is close to an ideal insulating ionic solid with a valence band structure dominated by the strong potential of the ionic cores. Studies of the electronic properties of MgO are motivated by its technological applications, such as in catalysis, microelectronics, and electrochemistry. Bulk MgO is relatively inert, but its reactivity is greatly enhanced in the nanoscale. The high surface area and the intrinsically high surface reactivity of MgO nanocrystals make these materials especially effective as adsorbents [6]. In fact, they have been called “destructive adsorbents” because of their tendency to adsorb and simultaneously destroy by bond breaking processes a series of toxic
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