Free energy of formation of clusters of sulphuric acid and water molecules determined by guided disassembly

TL;DR

This study uses a nonequilibrium molecular dynamics approach with guide particles to determine the free energy of formation of sulphuric acid-water clusters, providing insights into droplet nucleation processes.

Contribution

It introduces a novel guided disassembly method combined with the Jarzynski equality to evaluate cluster free energies in molecular dynamics simulations.

Findings

Reversible guide particle disassembly yields more accurate free energies.

Constructed a grand potential surface for sulphuric acid-water clusters.

Identified a critical cluster size for droplet nucleation.

Abstract

We evaluate the grand potential of a cluster of two molecular species, equivalent to its free energy of formation from a binary vapour phase, using a nonequilibrium molecular dynamics technique where guide particles, each tethered to a molecule by a harmonic force, move apart to disassemble a cluster into its components. The mechanical work performed in an ensemble of trajectories is analysed using the Jarzynski equality to obtain a free energy of disassembly, a contribution to the cluster grand potential. We study clusters of sulphuric acid and water at 300 K, using a classical interaction scheme, and contrast two modes of guided disassembly. In one, the cluster is broken apart through simple pulling by the guide particles, but we find the trajectories tend to be mechanically irreversible. In the second approach, the guide motion and strength of tethering are modified in a way that prises the cluster apart, a procedure that seems more reversible. We construct a surface representing the cluster grand potential, and identify a critical cluster for droplet nucleation under given vapour conditions. We compare the equilibrium populations of clusters with calculations reported by Henschel et al. [J. Phys. Chem. A 118, 2599 (2014)] based on optimised quantum chemical structures.