Models of cooperative diffusion

The intuitive idea of cooperative molecular rearrangement and diffusion, which is often invoked to rationalize experimental observations in liquids close to their glass transition, calls for dynamical models which lead to a clearer definition of the term 'cooperativity' and to a better understanding of its physical implications. Work on two lattice gas models, which were designed for this purpose, is reviewed: the hard-square-lattice gas and a cooperative triangular-lattice gas. Possible generalizations and some unsolved problems are pointed out.