Cluster formation, waterlike anomalies, and re-entrant melting for a family of bounded repulsive interaction potentials

We introduce a family of bounded repulsive potentials, which we call the cut ramp potential, obtained by cutting a linear ramp potential at different heights. We find that for the uncut ramp potential the system shows a region of anomalous re-entrant melting (a negative slope of the melting line in the temperature-pressure phase diagram), with waterlike anomalies in the same pressure range. At high pressure the melting line recovers a positive slope, a feature that we associate with the formation of clusters of particles separated by a more or less density-independent distance, the cluster separation, which is approximately equal to the ramp width sigma(1). As the ramp is cut at lower and lower heights, the region of anomalous behavior shrinks and eventually disappears while at the same time the formation of clusters becomes more favored, as it is energetically less unfavorable for particles to "climb up" the ramp. We relate the occurrence of anomalous behavior to the reduced efficacy of the soft repulsive length scale with increasing pressure. The clustering phenomenon partially restores this efficacy, giving rise to an approximately constant distance sigma(1) between the clusters. Our results may be useful to better understand the phase behavior of macromolecules as well as that of substances with nondirectional interactions that are capable of displaying liquid polymorphism.