A new Monte Carlo method for estimating free energy and chemical potential

A new Monte Carlo method of computing the free energy difference between two systems is presented. A universally usable optimal formula of the non-Boltzmann sampling is derived on a theoretical basis. The new technique is designed to sample both ensembles of interest at one simulation. Applications to the free energy difference between two different simple fluids and to the chemical potential of a Lennard-Jones fluid show that the new method is economical as well as reliable. The method can be extended to sampling several ensembles at a time.