  We present an algorithm for the computation of unbiased Green functions and
self-energies for quantum lattice models, free from systematic errors and valid
in the thermodynamic limit. The method combines direct lattice simulations
using the Blankenbecler Scalapino-Sugar quantum Monte Carlo (BSS-QMC) approach
with controlled multigrid extrapolation techniques. We show that the
half-filled Hubbard model is insulating at low temperatures even in the
weak-coupling regime; the previously claimed Mott transition at intermediate
coupling does not exist.
