Communication: Phase diagram of C36 by atomistic molecular dynamics and thermodynamic integration through coexistence regions

Abstract

We report an atomistic molecular dynamics determination of the phase diagram of a rigid-cage model of C36. We first show that free energies obtained via thermodynamic integrations along isotherms displaying “van der Waals loops,” are fully reproduced by those obtained via isothermal-isochoric integration encompassing only stable states. We find that a similar result also holds for isochoric paths crossing van der Waals regions of the isotherms, and for integrations extending to rather high densities where liquid-solid coexistence can be expected to occur. On such a basis we are able to map the whole phase diagram of C36, with resulting triple point and critical temperatures about 1770 K and 2370 K, respectively. We thus predict a 600 K window of existence of a stable liquid phase. Also, at the triple point density, we find that the structural functions and the diffusion coefficient maintain a liquid-like character down to 1400–1300 K, this indicating a wide region of possible supercooling. We discuss why all these features might render possible the observation of the melting of C36 fullerite and of its liquid state, at variance with what previously experienced for C60.

On the determination of phase boundaries via thermodynamic integration across coexistence regions

“…A cheap way to work out the entire phase diagram of a model fluid is by following the isothermal evolution of the pressure P as a function of density ρ, any loop in P (ρ) being the hallmark of the crossing of a two-phase region (see for instance Ref. [8,33,34]). However, traditional simulation methods usually fail to attain thermal equilibrium near transition points and the question is whether it is possible to obtain accurate transition thresholds from an equation of state which is only approximate in the coexistence regions.…”

“…[8,33,34]). However, traditional simulation methods usually fail to attain thermal equilibrium near transition points and the question is whether it is possible to obtain accurate transition thresholds from an equation of state which is only approximate in the coexistence regions.…”

“…The shortcoming shared by all these approaches is the necessity to assume a priori the knowledge of the crystalline structure, whose optimization may in fact be a rather daunting task if the system of interest is sufficiently complex (see, e.g., the cases analyzed in Refs. [7,8]). In such a case, metadynamics [9] or a genetic algorithm [10] may be useful ways out.…”

Shapes of a liquid droplet in a periodic box

“…In particular, the first inversion of concavity encountered corresponds to the first occurrence of liquid-vapor separation. We have recently showed that, despite their fake character, one can take advantage of these pressure loops to extract, by plain thermodynamic integration, the right liquid-vapor coexistence parameters [3,4].…”