Structural models for intermediate-sized Si clusters

We have investigated two generic structural models for Si clusters in the size range 20-33 atoms. The first model consists of stacks of puckered six-fold rings, with capping atoms on the two ends. The second model comprises compact structures with as many bulk-like four-fold coordinated atoms as possible. We have found the cohesive energy of the ring structures to be nearly constant as a function of cluster size, while that of the compact structures decreases dramatically with increasing size as more four-fold coordinated atoms are incorporated into the structures. We compare our results with recent experiments which suggest a crossover in stability between two cluster types in the size range studied here.