The box runs molecular dynamics: Newton’s laws for every atom, in small time steps. The force comes from the Lennard-Jones potential

U(r) = 4ε [ (σ/r)¹² − (σ/r)⁶ ]

The r⁻¹² term is a steep wall (atoms cannot overlap); the r⁻⁶ term is the weak van der Waals attraction. The minimum sits at r = 2^(1/6) σ ≈ 1.12 σ, with depth ε. With ε/k_B ≈ 120 K and σ ≈ 0.34 nm this describes argon well.