Obtaining equilibrium states in ultrasoft cluster forming systems using a combined thermo- and barostat.

By compressing a (two-dimensional) system of ultrasoft, cluster-forming particles via a combined thermo- and barostat, a cluster crystal is created that is obviously not in its equilibrium: launching from such a configuration expansion and compression runs leads to systems that differ in their density distinctively from the one of the initial state. With our analysis we can discard dynamic lag as the origin of these discrepancies and can confirm that they occur due to the complex interplay between the two mechanisms which accompany any change in volume of a cluster crystal, namely the variation of the lattice spacing and of the number of lattice sites. In our investigations we show that only combined melting and annealing runs are able to transform the system in its equilibrium. This is evidenced by the fact that the density of this state remains unchanged even after the application of combined compression- and expansion-experiments as well as combined melting- and annealing-runs; furthermore, the cluster crystal shows an essentially perfect hexagonal arrangement.