  We study the single-particle properties of quarter-filled ladder systems such
as sodium vanadate by means of a recently developed generalization of the
variational cluster perturbation theory to extended Hubbard models. We find a
homogeneous antiferromagnetic insulating phase for nearest-neighbor repulsions
smaller than a critical value, without any metallic phase for small repulsions.
Different from C-DMFT and LDA considerations, the inclusion of diagonal hopping
within a ladder has little effect on the bonding bands, while flattening and
shifting the antibonding bands. In the low-temperature charge-ordered phase,
the spectrum depends on whether the ordering is driven by the Coulomb repulsion
or by the coupling to a static lattice distortion. The small change of the
experimentally observed gap upon charge ordering implies that the lattice
coupling plays an important role in this ordering. Inter-ladder coupling is
straightforward to include within our method. We show that it has only a minor
effect on the spectral function. The numerically calculated spectra show good
agreement with experimental angle-resolved photo-emission data.
