Network structure representing the interactions between particles.
(a) A complete network with 20 nodes (particles). Each node connects to all others. (b) A ring network with 20 nodes. Each node links to its nearest two neighbors. (c) A scale-free network with 20 nodes, in which the node size represents the node degree, i.e. the number of edges associated with the node. It shows that most nodes have low degrees, yet there exist a few high-degree nodes (hubs).