The Role of Theory in Clusters Research

Ouster science is a relatively new area of research and lies at the borderline between physics and chemistry. Its interdisciplinary character follows simply from the fact that clusters are units of matter with properties intermediate between those of molecules and solids. This is also the main reason why the rationalization of the chemical and physical properties of clusters represents a formidable challenge for theory. In principle, one would like to be able to describe the transition from the molecular state to the bulk solid state within a single theoretical framework, using the same language and, possibly, the same degree of theoretical accuracy. 

One important aspect of clusters is the peculiar nature of these molecular entities. Although several cluster properties exhibit a rapid convergence to the 

Attempts to define a theoretical basis for the growth sequence of small clusters and thus to predict how they evolve into large metallic partides and finally form an extended system have met with little success. This is primarily due to the existence of competing growth mechanisms and of isomers with similar stalnlity. Furthermore, clusters whose structures form a section of a fee or a bcc lattice are known, as are clusters with pentagonal symmetry. Qearly, the two channels lead to mutually exclusive growth paths, either to cubic or icosahedral structures. 

The apparent completely different nature of the bonding between clusters like [M06O12] and [Os3(CO),2] makes the application of similar concepts or of a unifying view quite difficult. However, it is certainly possible to draw some general conclusions about the electronic character of the late transition metal clusters, that is, for clusters of metal atoms in low oxidation states. These systems also represent the largest fraction of the dusters reported so far. 

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The role of theory in cluster research is twofold. On the one hand, theory must be able to provide a basis for understanding the chemical and physical properties of small metal aggregates, whether naked or ligated, and to rationalize any observed trends. On the other hand, theory is also expected to furnish quantita-... 

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