  Dynamical quantum-cluster approaches, such as different cluster extensions of
the dynamical mean-field theory (cluster DMFT) or the variational cluster
approximation (VCA), combined with efficient cluster solvers, such as the
quantum Monte-Carlo (QMC) method, provide controlled approximations of the
single-particle Green's function for lattice models of strongly correlated
electrons. To access the thermodynamics, however, a thermodynamical potential
is needed. We present an efficient numerical algorithm to compute the grand
potential within cluster-embedding approaches that are based on novel
continuous-time QMC schemes: It is shown that the numerically exact cluster
grand potential can be obtained from a quantum Wang-Landau technique to
reweight the coefficients in the expansion of the partition function. The
lattice contributions to the grand potential are computed by a proper infinite
summation over Matsubara frequencies. A proof of principle is given by applying
the VCA to antiferromagnetic (short-range) order in the two-dimensional Hubbard
model at finite temperatures.
