Simple fluids with complex phase behavior

We find that a system of particles interacting through a simple isotropic potential with a softened core is able to exhibit a rich phase behavior including: a liquid-liquid transition in the supercooled phase, as has been suggested for water, a gas-liquid-liquid triple point, a freezing line with anomalous reentrant behavior. The essential ingredient leading to these features resides in the presence of two effective radii in the repulsive core. The potential investigated appears appropriate for a class of spherical polymeric micelles recently investigated.