Cluster dual fermion approach to nonlocal correlations

A general cluster dual fermion approach to nonlocal correlations in crystals is formulated. The scheme allows the treatment of long-range correlations beyond the cluster dynamical mean field theory and nonlocal effects in realistic calculations of multiorbital systems. The simplest approximation is shown to exactly correspond to the free-cluster dynamical mean field theory. This approach is applied to the one-dimensional Hubbard model. Already the first dual-fermion correction to the free cluster leads to a drastic improvement of the calculated Green's function.