A coherent site approximation for transport in disordered systems

For disordered systems in which the transition rate from a site to its neighbours depends only on the properties of the initial site, a coherent site approximation (CSA) is proposed as the appropriate homomorphic cluster coherent potential approximation. This CSA is ideally suited for treating systems in which transitions between localised states of different energy take place via excitation to a band of extended states. In the limits of very short and very long times, the results of the CSA coincide with those of the familiar continuous time random walk approximation and so provide a formal justification for the latter.