Phase separation of a multiple occupancy lattice gas

A binary lattice gas model that allows for multiple occupancy of lattice sites, inspired by recent coarse-grained descriptions of solutions of interacting polymers, is investigated by combining the steepest descent approximation with an exploration of the multidimensional energy landscape and by Gibbs ensemble Monte Carlo simulations. The one-component version of the model, involving on site and nearest-neighbour interactions, is shown to exhibit microphase separation into two sub-lattices with different mean occupation numbers. The symmetric two-component version of the multiple occupancy lattice gas is shown to exhibit a demixing transition into two phases above a critical mean occupation number.