What is Liquid ? [in two dimensions]

TL;DR

This paper explores defining, simulating, and characterizing two-dimensional liquids using atomistic computer simulations with various pair potentials, boundary conditions, and gravity to understand their properties and phase behavior.

Contribution

It introduces a practical simulation framework for two-dimensional liquids, incorporating boundary conditions and gravity, and validates results against established methods.

Findings

Gravity stabilizes the liquid-gas interface.

Longer-ranged potentials help stabilize the liquid phase.

Simulation results agree with previous phase diagram estimates.

Abstract

We consider the practicalities of defining, simulating, and characterizing "Liquids" from a pedagogical standpoint based on atomistic computer simulations. For simplicity and clarity we study two-dimensional systems throughout. In addition to the infinite-ranged Lennard-Jones 12/6 potential we consider two shorter-ranged families of pair potentials. At zero pressure one of them includes just nearest neighbors. The other longer-ranged family includes twelve additional neighbors. We find that these further neighbors can help stabilize the liquid phase. What about liquids? To implement Wikipedia's definition of liquids as conforming to their container we begin by formulating and imposing smooth-container boundary conditions. To encourage conformation further we add a vertical gravitational field. Gravity helps stabilize the relatively vague liquid-gas interface. Gravity reduces the messiness associated with the curiously-named "spinodal" (tensile) portion of the phase diagram. Our simulations are mainly isothermal. We control the kinetic temperature with Nos\'e-Hoover thermostating, extracting or injecting heat so as to impose a mean kinetic temperature over time. Our simulations stabilizing density gradients and the temperature provide critical-point estimates fully consistent with previous efforts from free energy and Gibbs' ensemble simulations. This agreement validates our approach.