Quasi-chemical theory with a soft cutoff

TL;DR

This paper introduces a soft-cutoff version of quasi-chemical theory for liquids, simplifying the treatment of smooth repulsive interactions and maintaining convergence properties, especially useful for realistic molecular simulations.

Contribution

It develops a soft-cutoff formulation of quasi-chemical theory, enhancing practical applicability for molecules with smooth repulsive interactions.

Findings

Soft-cutoff theory simplifies treatment of repulsive interactions.

Hard cutoffs still outperform soft cutoffs in convergence.

Method applicable to water and other liquids with smooth interactions.

Abstract

In view of the wide success of molecular quasi-chemical theory of liquids, this paper develops the soft-cutoff version of that theory. This development has important practical consequences in the common cases that the packing contribution dominates the solvation free energy of realistically-modeled molecules because treatment of hard-core interactions usually requires special purpose simulation methods. In contrast, treatment of smooth repulsive interactions is typically straightforward on the basis of widely available software. This development also shows how fluids composed of molecules with smooth repulsive interactions can be treated analogously to the molecular-field theory of the hard-sphere fluid. In the treatment of liquid water, quasi-chemical theory with soft-cutoff conditioning doesn't change the fundamental convergence characteristics of the theory using hard-cutoff conditioning. In fact, hard cutoffs are found here to work better than softer ones.