Tunnel model of liquid diffusion

A tunnel model of liquid diffusion, analogous to Barker's thermodynamic model, is constructed by separating particle motion into components parallel and perpendicular to a tunnel axis. By combining exact results for hard rod systems, one is able to predict both the tunnel diffusion coefficient and the corresponding velocity correlation function. The ratio of the tunnel diffusion coefficient to the Enskog value at the same density and temperature is obtained by matching the virial and the temperature. The value predicted lies outside the range investigated by Alder et al; therefore direct comparison to molecular dynamics results is not possible. However, the velocity correlation function has the negative region characteristic of dense fluids. It is suggested that the derivation of the tunnel model dynamics may be valid at close packed densities.