Molecular dynamics simulation of atomic clusters in equilibrium with a vapour

Properties of atomic or molecular clusters surrounded by a vapour phase are needed in predicting nucleation rates and in development of phenomenological nucleation models. We have performed molecular dynamic (MD) simulations to investigate effects of thermostatting, boundary conditions and system size in cluster–vapour equilibrium. The studied system consists of Lennard–Jones (LJ) argon atoms with a potential cutoff of 6s. We used both periodic boundary conditions and a spherical boundary with a repulsive wall to carry out the MD simulations. Interaction between the repulsive wall and vapour atoms disturbs both the density and temperature distributions of the vapour. The evolution of temperature was studied both without thermostat and with the system coupled to a Nosé–Hoover chain thermostat. We found that the particle exchange between the cluster and vapour phase is not able to equalize the temperature in constant-energy simulations. The Nosé– Hoover thermostat performs the temperature regulation quite well. First, following an equilibration period, compact cluster– vapour systems were simulated. Second, to study the effects of system size, the small system is embedded into a large vapour system with the same vapour density as the original system. The equilibrium cluster size and the thermodynamical properties such as surface tension, can quite accurately be predicted by using the small system with a limited number of vapour atoms, which shortens the simulation time.