Phase diagram of the Gaussian-core model

We revisit, with unprecedented accuracy, the phase diagram of the Gaussian-core model, a system of particles interacting via a Gaussian-shaped, purely repulsive potential. This model, originally introduced by Stillinger, yields an effective description of the statistical behaviour of dilute polymers; it is also known to exhibit a FCC-BCC transition at low densities and reentrant melting at high densities. We carried out extensive Monte Carlo simulations, complemented with exact free-energy calculations, and found that, contrary to previous knowledge: i) the fluid-FCC-BCC triple-point temperature is about one third of the maximum freezing temperature; ii) upon isothermal compression, the fluid exhibits a fluid-BCC-FCC-BCC-fluid sequence of phases in a narrow range of temperatures just above the triple point. We discuss these findings in relation to other softly repulsive potentials and to the phase behaviour of star-polymer solutions and charged colloids.