  We present a quantum Monte Carlo method which allows calculations on
many-fermion systems at finite temperatures without any sign decay. This
enables simulations of the grand-canonical ensemble at large system sizes and
low temperatures. Both diagonal and off-diagonal expectations can be computed
straightforwardly. The sign decay is eliminated by a constraint on the fermion
determinant. The algorithm is approximate. Tests on the Hubbard model show that
accurate results on the energy and correlation functions can be obtained.
