**What is Liquid? [in two dimensions]**

Travis Karl P. 1, Hoover William G. 2*, Hoover Carol G. 2, Hass Amanda B. 3

1 Department of Materials Science & Engineering

The University of Sheffield

Sheffield S1 3JD, United Kingdom2 Ruby Valley Research Institute

601 Highway Contract 60

Ruby Valley, Nevada 89833, USA

*E-mail: hooverwilliam@yahoo.com3 Department of Applied Maths

University of Leeds

Leeds LS2 9JT, United Kingdom

### Received:

Received: 12 March 2021; accepted: 24 March 2021; published online: 29 March 2021

### DOI: 10.12921/cmst.2021.0000009

### 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 Nosé-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.

### Key words:

liquids, molecular dynamics, phase equilibria, spinodals, statistical physics, tension

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