FREE-ENERGY CALCULATIONS FOR LENNARD-JONES SYSTEMS AND WATER USING THE EXPANDED ENSEMBLE METHOD - A MONTE-CARLO AND MOLECULAR-DYNAMICS SIMULATION STUDY

The method of expanded ensembles for calculation of free energy in Monte Carlo simulations is incorporated into molecular dynamics simulations. Calculations of the free energy for the Lennard-Jones system are carried out using both variants of the expanded ensemble method (i.e. Monte Carlo and molecular dynamics simulations) and are shown to give identical results. The free energy for two variants of the simple point charge water (rigid and flexible) is calculated using the Lennard-Jones system as a reference. Numerically very accurate results for the free energy are obtained. The results are in accordance with results obtained using other methods for calculation of free energy in computer simulations. The advantages of the presented method and possible applications for calculation of free energies for more complicated molecular systems are discussed.