  We present a novel approach to long-range correlations beyond dynamical
mean-field theory through a ladder approximation to dual fermions. The new
technique is applied to the two-dimensional Hubbard model. We demonstrate that
the transformed perturbation series for the nonlocal dual fermions has superior
convergence properties over standard diagrammatic techniques. The critical Neel
temperature of the mean-field solution is suppressed in the ladder
approximation, in accordance with quantum Monte-Carlo (QMC) results. An
illustration of how the approach captures and allows to distinguish short- and
long-range correlations is given.
