  We describe a strategy to improve the efficiency of free energy estimates by
reducing dissipation in nonequilibrium Monte Carlo simulations. This strategy
generalizes the targeted free energy perturbation approach [Phys. Rev. E. 65,
046122, 2002] to nonequilibrium switching simulations, and involves generating
artificial, "escorted" trajectories by coupling the evolution of the system to
updates in external work parameter. Our central results are: (1) a generalized
fluctuation theorem for the escorted trajectories, and (2) estimators for the
free energy difference Delta F in terms of these trajectories. We illustrate
the method and its effectiveness on model systems.
