Phase diagram of the Weeks-Chandler-Andersen potential from very low to high temperatures and pressures

A combination of two molecular simulation algorithms has been used to determine the solid-liquid coexistence of the Weeks-Chandler-Andersen (WCA) fluid from low temperatures up to very high temperatures. Values are reported for the coexistence pressure, temperature, energy, enthalpy change, and densities of both the liquid and solid phases. At very high temperatures, the coexistence pressure approaches the same 12th-power soft-sphere asymptote as the 12–6 Lennard-Jones potential. However, in contrast to the Lennard-Jones potential, which shows a discontinuity of pressure at low temperatures, the coexistence pressure of the WCA potential approaches the zero-temperature limit. Empirical relationships are determined to accurately reproduce the coexistence pressure and both solid and liquid phase densities from near zero temperature to very high temperatures. The simulation data are used to improve the accuracy of a WCA equation of state. The validity of common melting and freezing rules is tested.

Figure 1

Comparison of the solid-liquid coexistence (a) pressure, (b) liquid densities, and (c) solid densities for the WCA potential calculated in this work (●) with data from literature (*, [10]). The errors are approximately equal to the symbol size.

Figure 2

Comparison of the relative percentage difference in pressure (◼), liquid density (●), and solid density (▲) at different temperatures obtained in this work and data available from the literature [10].

Figure 3

Solid-liquid coexistence (a) pressure (●) and (b) freezing (upper points) and melting (lower points) densities (●) as a function of temperature.

Figure 4

Comparison of (a) r.d.d. and (b) f.d.c. WCA data as a function of temperature obtained in this work (●) with Lennard-Jones (○) data from the literature [15].

Figure 5

Comparison of (a) the overall pressure-temperature and the (b) pressure–low temperature behavior of the WCA fluid calculated in this work (●) with literature data [15] for the Lennard-Jones potential (---). The Lennard-Jones data were supplemented by calculations using Eq. (1) from Ref. [28].

Figure 6

Comparison of the solid-liquid coexistence pressure of the WCA potential (●) with data reported for the LJ potential (---) [15] as a function of reciprocal temperature.

Figure 7

Comparison of molecular simulation data for the compressibility factors of the WCA fluid obtained in this work (●) with calculations using the Heyes and Okumura (○), Verlet and Weis EOS (◻), and Kolafa and Nezbeda (▽) equations of state. The solid line represents calculations of the reparametrized Heyes and Okumura equation reported here.

Figure 8

(a) Comparison of radial distribution functions at $T^{\ast }=1.0$ for the WCA fluid at freezing (solid line) and melting (dashed line) points. (b) A typical structure factor curve for the WCA fluid at a freezing point (${\rho }^{\ast }=0.98$, $T^{\ast }=1.15$).

Figure 9

Comparison of entropy of fusion obtained in this work (●) for the WCA fluid with the data for the Lennard-Jones potential (○) [15].