Free-energy calculation for rotating systems using the Monte Carlo method

The theoretical calculation of the potential of mean force is carried out for rotating systems with constant angular momentum, and described in terms of the effective rovibrational potential-energy surface. To estimate free-energy differences along an arbitrary reaction coordinate ξ(R), we develop two different numerical approaches, using either the “blue-moon” ensemble sampling constraining the system to a fixed value of ξ, or the well-known umbrella sampling technique. Both methods are implemented using the Monte Carlo scheme. As an illustration, we calculate the mean force between two Ar13 clusters versus the intercluster distance, including centrifugal effects.