Escorted free energy simulations: Improving convergence by reducing dissipation

Abstract

Nonequilibrium, "fast switching" estimates of equilibrium free energy differences ΔF are often plagued by poor convergence due to dissipation. We propose a method to improve these estimates by generating trajectories with reduced dissipation. Introducing an artificial flow field that couples the system coordinates to the external parameter driving the simulation, we derive an identity for ΔF in terms of the resulting trajectories. When the flow field effectively escorts the system along a near-equilibrium path, the free energy estimate converges efficiently and accurately. We illustrate our method on a model system and discuss the general applicability of our approach. © 2008 The American Physical Society.