Thermal transitions in small water clusters

Small water clusters absorb heat radiation and participate in pivotal atmospheric reactions.

While (H₂O)_n clusters (n = 3- 5) are cyclic at low temperatures, their structures at higher temperatures were not directly probed. Diverse estimates for the “melting temperature” of water clusters were obtained from molecular dynamics, yet its molecular origins have not been established, and the insufficient quality of the potential energy surfaces (PES) casts doubts on the obtained values. Moreover, an analogous “vaporization temperature” has not been reported, so it remains unclear if any of these clusters remains intact in the atmospheric temperature range (≈220-290 K).

We have utilized a most accurate water potential (MB-pol) from the Paesani group together with the accurate g-BAOAB integrator, to run isothermal classical molecular trajectories for small water clusters (n =2-6), revealing a plethora of conformational changes with different onset temperatures. However, even when a high-quality PES is utilized, the conventional determination of “melting temperatures” using the Lindemann criterion remains rather arbitrary. We have found how to generalize it to reveal the rare events occurring at the transition thresholds. In particular, we determine both the melting and (for the first time also) the vaporization temperatures for these clusters and establish their molecular origins. Interestingly, all clusters are expected to dissociate below the atmospheric temperature range, except for the water tetramer, which could therefore be the workhorse for several significant atmospheric reactions.