Density anomaly in a competing interactions lattice gas model

Water and other tetrahedral liquids are characterized by a density anomaly whose origin is not well understood. A very simple model of a short-range attraction followed by an outer shell repulsion is proposed as a test potential for the density anomaly. We show that these competing interactions when applied to a two-dimensional lattice gas leads to the formation of two liquid phases and to the appearance of a density anomaly. The coexistence line between the two liquid phases meets a critical line between the fluid and the low-density liquid phase at a tricritical point. The line of maximum density emerges in the vicinity, of the tricritical point, close to the demixing transition.