Nanoparticles, which are aggregates of identical or different atoms or molecules ranging from a few to many millions, are called atomic or molecular clusters [1-6]. They constitute a separate class of materials in between the bulk on one hand and the molecular state on the other. Their chemical, optical, electrical, physical, and magnetic properties can be signi–cantly different from the respective bulk materials. For instance, while bulk gold appears yellow in color, nanosized gold particles appear in red, or while bulk gold does not react with oxygen, gold clusters do. Thus, materials that are commonly chemically inert in bulk can become catalytically active , nonmagnetic bulk materials can show magnetic behavior when scaled down to nanostructures , and metallic materials can become semiconductors . The main reasons for such property changes are as follows: in the regime of clusters, gravitational forces become negligible and electromagnetic forces begin to dominate, quantum mechanical effects dominate the motion and energy instead of the classical mechanics, the surface-to-volume ratio becomes much greater, and random molecular motion becomes more important. In contrast to molecules, nanoclusters do not have a –xed size or composition. For example, the vitamin C molecule (see Figure 10.1a) contains six oxygen, six carbon, and eight hydrogen atoms, which are placed at certain positions. On the other hand, platinum clusters (see Figure 10.1b and c) may contain any number of constituent particles and present a variety of morphologies for a –xed size.