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
