An efficient kinetic Monte Carlo scheme for computing Helmholtz free energy and entropy in bulk fluids and adsorption systems

Abstract

We present an efficient kinetic Monte Carlo scheme to determine the Helmholtz free energy and entropy of bulk fluids and adsorption systems. The method is made possible because this technique enables the accurate determination of the chemical potential. The Helmholtz free energy, A, is obtained by integrating the chemical potential with respect to the number of molecules, at constant volume and temperature, and the entropy is then determined from the fundamental thermodynamic equation A = E − TS. The entropy of bulk argon is found to be in excellent agreement with values calculated from the established equation of state (EOS). In a system with two co-existing phases we show that our method can determine the surface tension at the Gibbs dividing surface, without recourse to the mechanical route of Irving, Kirkwood and Buff.We show that the intrinsic integral molecular Helmholtz free energy and entropy of the adsorbed phase, corrected for the surface excess, are independent of the size of the simulation box for all chemical potentials tested.The new procedure is illustrated for a range of gases commonly used in the characterization of porous solids (argon, nitrogen, carbon dioxide and ammonia) as adsorbates and a graphitic slit pore as the model adsorbent.