Efficient computation of the free-space periodic Green´s function

The application of Shanks´s transform is shown to improve the convergence of the series representing the doubly infinite free-space periodic Green´s function. Higher order Shanks transforms are computed via Wynn´s epsilon algorithm. Numerical results confirm that a dramatic improvement in the convergence rate is obtained for the on-plane case, in which the series converges extremely slowly. In certain instances, the computation time can be reduced by as much as a factor of a few thousands. A relative error measure versus the number of terms taken in the series is plotted for various values of a convergence factor as the observation point is varied within a unit cell. Computation times are also provided.