Can we rationalize the structure of small silicon-carbon clusters?

A theoretical study of SimCn clusters with m + n ∈ 〈3,6〉 using density functional theory is presented. Tests of various functionals demonstrate that local spin density approximation (LSDA) is the most adequate functional for the study of these systems. Structures, vibrational frequencies, and IR intensities of the lowest energy isomer of the studied clusters obtained using LSDA are described, and the unusual properties of the Si-C clusters are discussed. A quantitative analysis of the obtained structures was carried out, and relations between the coordinations, interatomic distances, and angles observed in the Si-C clusters were obtained through introduction of the notion of coordination. This analysis also shows that the carbon atoms mainly exhibit sp and sp2 hybridizations, and that a majority of silicon atoms do not hybridize. This study is the first step of the implementation of a semi-empirical potential, which would describe the moderately small Si-C clusters. © EDP Sciences Springer-Verlag 1998.