Metadynamics molecular dynamics and isothermal Brownian-type molecular dynamics simulations for the chiral cluster Au18

Abstract

In an effort to gain insight into enantiomeric transitions, their transition mechanism, time span of transitions and distribution of time spans etc, we performed molecular dynamics (MD) simulations on chiral clusters Au10, Au15 and Au18, and found that viable reaction coordinates can be deduced from simulation data for enlightening the enantiomeric dynamics for Au10 and Au15, but not so for Au18. The failure in translating the Au18-L ⇌ Au18-R transitions by MD simulations has been chalked up to the thermal energy k B T at 300 K being much lower than energy barriers separating the enantiomers of Au18. Two simulation strategies were taken to resolve this simulation impediment. The first one uses the well-tempered metadynamics MD (MMD) simulation, and the second one adeptly applies first a somewhat crude MMD simulation to locate a highly symmetrical isomer Au18S and subsequently employed it as initial configuration in the MD simulation. In both strategies, we work in collective variable space of lower dimensionality. The well-tempered MMD simulation tactic was carried out aiming to offer a direct verification of Au18 enantiomers, while the tactic to conduct MMD/MD simulations in two consecutive simulation steps was intended to provide an indirect evidence of the existence of enantiomers of Au18 given that energy barriers separating them are much higher than ca. k B T at 300 K. This second tactic, in addition to confirming indirectly Au18-L and Au18-R starting from the symmetrical cluster Au18S, the simulation results shed light also on the mechanism akin to associative/nonassociative reaction transitions.