Phase behavior of a three-dimensional core-softened model system

The phase behavior of a three-dimensional model substance interacting via a continuous core-softened pair potential is studied. A combination of thermodynamic integration and free-energy augmented metadynamics is employed to identify the ground state structure as simple hexagonal. A transition to close packing is predicted at high pressure. The melting line is traced, which when combined with the liquid-vapor line, allows the liquid to be studied over its entire thermodynamically stable range. We find no liquid anomalies in the range of pressures for which the vaporization transition exists. Our results therefore support suggestions that the anomalous behavior of such fluids is unique to the two-dimensional case