Shape of small silicon clusters

We propose an explanation for the experimentally observed transition in the shape of silicon clusters of size 20\ensuremath{\le}N\ensuremath{\le}40: Elongated shapes of low energy can be obtained by stacking stable subunits, while concurrent optimization of surface-to-volume ratio and surface structure leads to compact shapes. A transition in shape from elongated to compact structures is expected as the size increases beyond a critical value at which interior atoms become stable. Our proposal is backed by extensive first-principles calculations on the energetics of two classes of Si clusters, which suggest a critical size bounded by 24\ensuremath{\le}N\ensuremath{\le}28, in good agreement with experimental observations.