Clusters are of interest for a number of reasons.
Firstly, they lie between the quantum regime of small molecules with discrete energy levels and the classical regime of condensed matter where states are continuous. The study of the evolution of the geometric and electronic structures of clusters and their chemical and physical properties is also of great interest and can aid our understanding of a whole range of topics in condensed matter science. Finally, the high ratio of surface to interior atoms in clusters means that there are many points in common between the chemistry and physics of clusters and the surfaces of solids and liquids. The study of the adsorption and reaction of small molecules on the surfaces of transition metal clusters continues to attract much attention because of its relevance to the field of heterogeneous catalysis. Theory plays an important role in cluster science.