Vapor-liquid coexisting morphology of all-atom water model through generalized isothermal isobaric ensemble molecular dynamics simulation

Abstract

Exploring the atom-scale details such as morphology of coexisting phase during phase transitions is very important for understanding their microscopic mechanism.While most theories,such as the classic nucleation theory,usually over-simplify the character of the critical nucleus,like the shape,structure,and most current experiment techniques are hardly to capture the instantaneous microscopic details,the atomistic molecular dynamics (MD) or Monte Carlo (MC) simulation provides a promise to detect the intermediate process of phase transitions.However,the standard canonicalensemble MD/MC simulation technique can not sufficiently sample the instantaneous (unstable in thermodynamics) coexistent phase.Therefore,the MC in the general canonical ensemble,such as general isothermal-volume ensemble (gNVT),combined with the enhanced sampling techniques,such as the replica exchange (RE) method,was presented to stabilize then to sufficiently sample the atomic conformations of the phase coexistence.Due to the limit of the RE, the RE-MC simulation on gNVT is usually applied in smaller systems.In this paper,we first extend the gNVT-based MC simulation to the MD in the generalized isothermal-isobaric ensemble (gNPT) and very simply implement it in the standard atomic MD soft packages without modifying the code,so that we can use these packages in MD simulation of realistic systems.Then we simulate the vapour-liquid phase transition of all-atomic water model.At least at not very low pressures,we find that the individual gNPT simulation is already enough to reach equilibrium in any region of the phase transition,not only in the normal liquid and vapour regions,but in the super-saturation regions,and even in the vapour-liquid coexistent regions.The obtained energy-temperature curve in the cooling gNPT well matches with that in the heating procedure without any hysteresis.It indicates that it is not necessary to use the RE technique in the gNPT,and the intermediate states during phase transitions in larger systems can be effectively simulated by a series of independent individual gNPT-MD simulations in the standard soft packages.We also propose a method to accurately determine the interface between the two phases in the coexistence,then provide a quantitative measurement about the interface tension and the morphology of the coexistent phase in the larger all-atomic water at various temperatures and pressures.The results show that the liquid droplet (or vapour bubble) at the low pressure is close to a sphere due to the larger interface tension,as expectation of the classic nucleation theory of the first-order phase phase transition,but becomes more and more irregular as the decrease of the interfacial tension as increasing the pressure to approach to the critical pressure,where the phase transition is the second order one.