Methods for calculating the free energy of atomic clusters

Abstract

We have extended the finite-time variational method for calculating free energy differences, DeltaA, to atomic clusters. The main obstacle in calculating DeltaA for finite systems is finding a suitable reference state. We present an optimal reference state for liquid clusters as well as one for solid clusters. Thus, the free energy for clusters in each phase can be calculated separately The method is tested on LJ55 for a wide range of temperatures. This method should be easily extended to molecular clusters or large molecules and would allow one to determine the stability of different geometrical isomers at non-zero temperatures. We also examine the commonly used practice of adjusting the maximum step size during Metropolis Monte Carlo simulation.