Thermal motion in a cluster of 14 argon atoms. Notice how 13 of these atoms form a compact core, and the 14th atom is weakly bound at the periphery. The liquid nature of this cluster is clearly demonstrated in the picture, which shows atoms freely diffusing through the cluster (time series: 1, 2, and 3). In particular, note how peripheral atoms (gold in 1 and purple in 2) do not stay peripheral. Above the critical temperature, the cluster boils into individual particles.
* shows the cluster's lowest energy configuration. In this panel, gold spheres lie at the vertices of two pentagons in parallel planes.