Size-dependent phase changes in water clusters

We investigate melting behavior of water clusters (H₂O) _N (N = 7, 8, 11, and 12) by using multicanonical-ensemble molecular dynamics simulations. Our simulations show that the melting behavior of water clusters is highly size dependent. Based on the computed canonical average of the potential energy and heat capacity C_V, we conclude that (H ₂O)₈ and (H₂O)₁₂ exhibit first-order-like phase change, while (H₂O)₇ and (H ₂O)₁₁ exhibit continuous-like phase change. The melting temperature range for (H₂O)₈ and (H₂O) ₁₂ can be defined based on the peak position of C_V(T) and dC_V(T)/dT (where T is the temperature). Moreover, for (H ₂O)₈ and (H₂O)₁₂, the solid- and liquid-like phases separate temporally in the course of simulation. In contrast, no temporal separation of solid- and liquid-like phases is observed for (H ₂O)₇ and (H₂O)₁₁. In light of the notable temporal separation of solid- and liquid-like phases for(H ₂O)₈ and (H₂O)₁₂, an alternative computer approach for estimating the melting temperature range is proposed based on the time-dependent Lindemann parameters. We find that the melting temperature range estimated from both definitions is consistent with each other for (H₂O)₈ and (H₂O)₁₂ but not for (H₂O)₇ and (H₂O)₁₁. We also find that the melting behavior of small water clusters can be conveniently assessed if the energy differences of neighbor-sized clusters at zero temperature are known.