Estimating equilibrium properties from non-Hamiltonian dynamics

We derive an expression that enables the accurate estimation of equilibrium properties using non-Hamiltonian dynamics. The major advantage of our sche me is that a time average over a single non-Hamiltonian trajectory can be e mployed instead of an ensemble average. Hence, it can directly be used in s tandard molecular dynamics simulations. The connection between non-Hamilton ian dynamics and equilibrium properties is established by assigning to the individual frames of the trajectory a weight that is based on the fluctuati ons of the phase space compression factor. Additionally, a simple scheme th at takes into account only fluctuation of a given maximum duration is intro duced to reduce the statistical error. By systematically extending the dura tion of the allowed fluctuations, increasingly accurate results can be obta ined. Non-Hamiltonian dynamics schemes that are capable to enhance sampling efficiency are applied to two model systems in order to demonstrate the pr actical performance of our approach for the calculation of equilibrium free energy differences and probability density profiles. (C) 2001 American Ins titute of Physics.