The EXP pair-potential system. III. Thermodynamic phase diagram

This paper determines the thermodynamic phase diagram of the EXP system of particles interacting by the purely repulsive exponential pair
potential. The solid phase is face-centered cubic (fcc) at low densities and pressures. At higher densities and pressures, the solid phase is bodycentered cubic (bcc) with a re-entrant liquid phase at the highest pressures simulated. The investigation first identifies the phase diagram at
zero temperature at which the following four crystal structures are considered: fcc, bcc, hexagonal close packed, and cubic diamond. There is a
T = 0 phase transition at pressure 2.651 × 10−3 with the thermodynamically stable structure being fcc below and bcc above this pressure. The
densities of the two crystal structures at the phase transition are 1.7469 × 10−2
(fcc) and 1.7471 × 10−2
(bcc). At finite temperatures, the fcc–
bcc, fcc-liquid, and bcc-liquid coexistence lines are determined by numerical integration of the Clausius–Clapeyron equation and validated
by interface-pinning simulations at selected state points. The bcc-fcc phase transition is a weak first-order transition. The liquid-fcc–bcc triple
point, which is determined by the interface-pinning method, has temperature 5.9 × 10−5
and pressure 2.5 × 10−6
; the triple-point densities
are 1.556 × 10−3
(liquid), 1.583 × 10−3
(bcc), and 1.587 × 10−3
(fcc).