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 Mon te Carlo simulations is incorporated into molecular dynamics simulatio ns. Calculations of the free energy for the Lennard-Jones system are c arried out using both variants of the expanded ensemble method (i.e. M onte Carlo and molecular dynamics simulations) and are shown to give i dentical results. The free energy for two variants of the simple point charge water (rigid and flexible) is calculated using the Lennard-Jon es system as a reference. Numerically very accurate results for the fr ee energy are obtained. The results are in accordance with results obt ained using other methods for calculation of free energy in computer s imulations. The advantages of the presented method and possible applic ations for calculation of free energies for more complicated molecular systems are discussed.