Isomerization rates and mechanisms for the 38-atom Lennard- Jones cluster determined using molecular dynamics and temperature accelerated molecular dynamics

We have simulated the isomerization of 38-atom Lennard-Jones clusters (LJ38) – a well-known prototype system for global-optimization and energy landscape analyses – over a wide range of temperatures using a combination of molecular dynamics (MD) and temperature accelerated dynamics (TAD). At high temperatures, we find that (surprisingly) MD with steepest descent quenches (MDSDQ) is effective for finding the global minimum of LJ38 as well as for determining the isomerization rates between the stable fcc isomer and the group of metastable icosahedral isomers. Importantly, the MDSDQ-determined isomerization rate was found to be approximately seven times greater than in previous studies based on discrete path sampling (DPS). The long simulation times enabled by TAD allows for the determination of isomerization rates at low temperatures, and reveals that isomerization proceeds primarily via processes with relatively low activation barriers. Using TAD we have found a new LJ38 isomerization pathway with a smaller activation barrier than any pathway published previously. In addition, we have determined that correlated events play an important role in the accurate determination of isomerization rates. We conclude that the slower rates observed by DPS are due to the omission of correlated events and some low energy pathways.