Anomalous melting and solid polymorphism of a modified inverse-power potential

Abstract

We numerically investigate a system of particles interacting through a repulsive pair potential of inverse-power form, modified in such a way that the strength of the repulsion is softened in a range of distances. The solid phases of the system for various levels of softness are identified by computing the zero-temperature phase diagram; then, for each solid phase, the melting line is determined by Monte Carlo simulation. Upon increasing the softness of the potential core, a region appears where melting occurs upon compression at constant temperature (‘anomalous’ melting) and a number of low-coordinated crystals become stable at moderate pressures. Next, the structural properties of the system for varying core softness are surveyed in the hypernetted-chain approximation, whose accuracy has been positively tested against numerical simulation. For sufficiently high degrees of softness, the radial distribution function shows the typical interplay between two distinct length-scales. In a narrow range of moderate softness, reentrant melting occurs instead with just one length-scale, which shows that the existence of two well-definite length-scales is not the only mechanism for anomalous melting.