TABLE OF CONTENTS Atomic clusters are aggregates of atoms containing from few to a few thousand atoms. Due to their small size, the properties of the clusters are, in general, different from those of the corresponding material in the macroscopic bulk phase. The differences result from the fact that the number of atoms forming the surface is a substantial fraction of the number of atoms forming the cluster, while this fraction is negligible in the case of a macroscopic solid (of course the surface of a solid is relevant in itself, giving rise to the technologically important field of surface science). Many of the differences between clusters and bulk can also be viewed as arising from the small volume of the potential well confining the electrons in the clusters. In this case the electrons fill discrete levels, instead of having the continuous distribution (bands) characteristic of the solid. Of course the two views are inter-related. By studying the properties of clusters, scientists expect to obtain information on the early stages of growth of matter, and on the evolution of the properties towards the bulk. An interesting question which still lacks a convincing answer in many cases is the following: how many atoms are required for a cluster to show the properties of the bulk material? To make affairs more complicated, sometimes different properties of a given type of clusters appear to converge at a different rate. But, even more important, is knowing the precise behavior of a given...
