Eliminating the minus sign in Monte Carlo simulations of fermions.

Practically all the Monte Carlo algorithms for identical fermions require the substraction of separate contributions arising from, respectively, even and odd permutations of the particles. At low tempertures this becomes a serious problem since the two contribution nearly cancel. A new method is proposed for obtaining this difference directly. This amounts to a constraint that limits configuration space to a region bounded by the nodal surface of the fermionic ground state wave function. A detailed description of the method is given for the general real hamiltonian in the continuum where the partition function paths are approximated by "polymers" of finite length. The only departure from the usual bosonic treatment is the use of a nonlocal constraint which prohibits the polymer from simulationeously occupying regions of configuration space related by an odd permutation.