Convergent real-space cluster expansion for configurational disorder in ionic systems

We present a rapidly converging, real-space expansion to compute the electrostatic energy of point-charge configurations on a fixed lattice. The rapid convergence is obtained by requiring that the expansion only reproduce well the configurations with energies below some cutoff energy. The convergence rate can be systematically varied by changing this cutoff. This expansion should prove useful for computations in which only low-energy states are required, such as free-energy and phase-diagram computations.