Free energy via thermostatted dynamic potential-energy changes

Three dynamical methods for computing canonical-ensemble Helmholtz free energies are discussed and compared for a thermostatted six-body harmonic chain. We use a Martyna-Klein-Tuckerman thermostat [J. Chem. Phys. 97, 2635 (1992)] with six time-reversible friction coefficients to study both single-trajectory and ensemble-averaged free-energy changes. Though all three dynamic methods produce identical long-time averages, the one based on Kirkwood's coupling-parameter theory [J. Chem. Phys. 3, 300 (1935)] converges much more rapidly than do the two based on time-integrated heat transfer.