Polymorphism, thermodynamic anomalies and network formation in an atomistic model with two internal states

Using molecular dynamics simulations we study the temperature-density phase diagram of a simple model system of particles in two dimensions. In addition to translational degrees of freedom, each particle has two internal states and interacts with a modified Lennard-Jones potential which depends on relative positions as well as the internal states. We find that, despite its simplicity, the model has a rich phase diagram showing many features of common network-forming liquids such as water and silica, including polymorphism and thermodynamic anomalies. We believe our model may be useful for studies concerning generic features of such complex liquids.