  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.
