Fermion simulations in systems with negative weights

We investigate the use of two methods - Langevin and molecular dynamics - for simulating fermion systems with real, non-positive definite probability functions. The methods are tested on a simple one-dimensional model. Both methods have difficulties associated with the nodes of the probability function. While the Langevin equation can cross the nodes even for time steps as small as 10-10, the crossing may be so violent that an equilibrium distribution is not readily achieved. The molecular dynamics system is trapped within regions bounded by the nodes, not sampling all of phase space. A hybrid Monte Carlo-molecular dynamics method is proposed which avoids these difficulties in the Langevin and molecular dynamics techniques. 16 refs