(178i) Towards More Efficient Enhanced Sampling Methods to Study Phase Transitions

Rare events are difficult to simulate using conventional molecular simulations. In this work, we show how we can circumvent this issue by using efficient enhanced sampling methods such as the umbrella sampling technique and the Wang-Landau sampling method. In particular, we study phase transitions between liquid-solid, gas-solid and gas-liquid phases. Using molecular simulation with the umbrella sampling technique, we study the molecular mechanisms underlying the nucleation and growth of C60 nanoparticles from the supersaturated vapor and from the undercooled liquid. We show that in both cases, nucleation proceeds through the formation of small clusters composed of the metastable hexagonal close-packed (HCP) polymorph. This observation differs from the nucleation pathway of simple systems  (e.g. modeled by Lennard-Jones potential) in which the nucleation proceeds through the metastable body-centered cubic (BCC) polymorph. For the gas-liquid transition, we combine hybrid Monte Carlo simulations with the Wang-Landau sampling technique to study the gas-liquid transition in a series of polycyclic aromatic hydrocarbons.