Chemical potential, partial enthalpy and partial volume of mixtures by NPT molecular dynamics

Abstract

Equilibrium NPT molecular dynamics computer simulations have been used to determine the chemical potential, partial enthalpy and partial volume of model Ar/Kr-like mixtures using the non-intrusive particle insertion and particle swap techniques devised by P. Sindzingre et al. [Chem. Phys. 129 (1989) 213]. Both species are represented by single-site Lennard-Jones (LJ) pair potentials with Lorentz-Berthelot rules for the cross-species interactions. In this report we use these techniques to examine further (a) the relative convergence statistics of the particle swap and particle insertion methods for these properties for model argon-krypton mixtures, and (b) to investigate the effect of changing the relative values of the LJ well-depth, ϵ, and core-diameter, σ, for the two species. If the two species in the binary mixture are labelled A and B, we show that species A → B virtual particle swaps and B → A swaps lead to statistically identical values for the difference in the chemical potentials, partial enthalpies and partial volumes between the two species. This confirms that no bias is introduced in the sampling of phase space by this method. Over the whole phase diagram and especially in the vicinity of the fluid-solid coexistence line, the particle swap method converges significantly more rapidly than the particle insertion method for the difference in chemical potential of the two species, partial enthalpy and partial volume of each species. Also, we find that, using the particle swap method, the difference in the chemical potential converges more rapidly than the differences in the partial enthalpy and volume. We use the swap technique to investigate the effects of changing the molecular potential parameters on the partial properties. We consider artificial binary mixtures con- structed from various choices of the σ Ar, σ Kr, ϵ Ar and ϵ Kr given to the two species. We discover that the effects of the argon and krypton LJ parameters largely cancel for the mixture thermodynamic properties. Arbitrary combinations of these parameters can give rise to more extreme composition dependence for the average and partial properties. The partitioned and partial enthalpies provide a useful probe of the local structural changes accompanying probe particle insertion.