Thermodynamic and structural properties of model systems at solid-fluid coexistence

Abstract

Coexistence between the fluid and solid phases of systems modelled by the inverse-power potential φ(r) = ε(σ/r) n is studied as a function of the potential softness s ≡ 1/n. Beginning from the fcc fluid coexistence point of hard spheres (s = 0), the Gibbs-Duhem integration molecular simulation technique is applied to trace out the solid-fluid transition pressure (reciprocal temperature) in increments of 0·01 in s, reaching a maximum s = 0·33. System sizes of the order of 500 spheres are used to model each phase, and a systematic study of finite-size effects is not attempted; thus results for s nearing its maximum are only tentative. Significant disagreement is seen with the results of early studies of inverse-power systems (for n = 12, 9, 6, and 4), while confirmation of more recent data (for n = 12 and 6) is found. Freezing into a bcc crystal is also investigated, and it is estimated that for softness s > 0·16 the bcc phase is the stable one at freezing. Coexistence between the fcc and bcc phases is not examined beyond locating the ‘triple point’ of fcc-bcc-fluid equilibrium. Several semi-empirical ‘melting rules’ are examined in the light of the new data.