Abstract
A large number of phenomena of scientific and technological interest involve multiple phases and occur at constant pressure of one of the two phases, e.g., the liquid phase in vapor nucleation. It is therefore of great interest to be able to reproduce such conditions in atomistic simulations. Here we study how popular barostats, originally devised for homogeneous systems, behave when applied straightforwardly to heterogeneous systems. We focus on vapor nucleation from a super-heated Lennard-Jones liquid, studied via hybrid restrained Monte Carlo simulations. The results show a departure from the trends predicted for the case of constant liquid pressure, i.e., from the conditions of classical nucleation theory. Artifacts deriving from standard (global) barostats are shown to depend on the size of the simulation box. In particular, for Lennard-Jones liquid systems of 7000 and 13 500 atoms, at conditions typically found in the literature, we have estimated an error of 10-15 kBT on the free-energy barrier, corresponding to an error of 104-106 s-1σ-3 on the nucleation rate. A mechanical (local) barostat is proposed which heals the artifacts for the considered case of vapor nucleation.
Highlights
Atomistic simulations are routinely used to investigate a variety of multiphase nanoscale systems, such as bubbles, drops, solid walls in contact with fluids, and solutions
We considered a system composed of particles interacting via the truncated and force shifted (TFS) LennardJones (LJ) potential, analogous to those considered in Refs. 7 and 8, uTFS(rij) = uLJ − uLJ −
We have addressed the issue of controlling pressure in vapor nucleation from a metastable liquid
Summary
Atomistic simulations are routinely used to investigate a variety of multiphase nanoscale systems, such as bubbles, drops, solid walls in contact with fluids, and solutions. We focus on the homogeneous nucleation of a vapor bubble in a metastable liquid This deceptively simple case allows us to analyze the shortcomings of standard barostats in dealing with multiphase systems domains at different pressures. This model is based on a number of approximations, including the fact that the interface is ideally sharp, that are sometimes violated in actual systems Within these approximations, it allows us to obtain an explicit dependence of the liquid pressure and of the energetics of the process on the volume of the vapor bubble, which helps understanding the shortcomings of standard (global) barostats. The liquid volume is assumed to be constant during nucleation and consistent with the bulk density of atomistic systems of N = 7000 and 13 500 particles: V L = N/ρL, where ρL = 0.58 is the metastable liquid density at the current pressure and temperature of simulations. Consistently with the macroscopic sharp-interface model in Eq (1), when the interface thickness is negligible, Eq (6) reduces to the volume-weighted average of the liquid and vapor pressures P (V L/V )PL + (V P/V )PV
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