Local free energy approximations for the coarse-graining of adsorption phenomena

TL;DR

This paper introduces a new coarse-graining method for adsorption systems using local free energy approximations based on the Bethe-Peierls approach, improving accuracy at high densities without separate pore simulations.

Contribution

It proposes a Bethe-Peierls based coarse-graining strategy that computes interaction terms via local effective potentials, enhancing accuracy over traditional Monte Carlo methods.

Findings

More accurate equilibrium property reproduction at high sorbate densities

Effective interaction parameters computed without separate pore simulations

Validated on lattice-gases and Lennard-Jones particle systems

Abstract

We investigate the coarse-graining of host-guest systems under the perspective of the local distribution of pore occupancies, along with the physical meaning and actual computability of the coarse-interaction terms. We show that the widely accepted approach, in which the contributions to the free energy given by the molecules located in two neighboring pores are estimated through Monte Carlo simulations where the two pores are kept separated from the rest of the system, leads to inaccurate results at high sorbate densities. In the coarse-graining strategy that we propose, which is based on the Bethe-Peierls approximation, density-independent interaction terms are instead computed according to local effective potentials that take into account the correlations between the pore pair and its surroundings by means of mean-field correction terms, without the need of simulating the pore pair separately. Use of the interaction parameters obtained this way allows the coarse-grained system to reproduce more closely the equilibrium properties of the original one. Results are shown for lattice-gases where the local free energy can be computed exactly, and for a system of Lennard-Jones particles under the effect of a static confining field.