Ring kinetic theory for tagged-particle problems in lattice gases

The kinetic theory for tagged-particle problems in lattice-gas cellular automata is extended beyond Boltzmann's mean-field approximation by including correlated ring-type collisions. This theory provides explicit expressions for the velocity autocorrelation function (VACF) for all times in terms of the ring-collision integral, as well as corrections to the Boltzmann values of the transport coefficients. For times long compared to the mean free time, the ring integral equation yields the phenomenological mode-coupling theory and the long-time tails. For intermediate times it describes a slow transition from initial exponential decay to the long-time tails. At short times the ring kinetic theory is exact. In particular, deviations from the Boltzmann result in the VACF of three-dimensional systems after two time steps are calculated explicitly and compared with computer simulations