  The excess free energy of a molecular cluster is a key quantity in models of
the nucleation of droplets from a metastable vapour phase; it is often viewed
as the free energy arising from the presence of an interface between the two
phases. We show how this quantity can be extracted from simulations of the
mechanical disassembly of a cluster using guide particles in molecular
dynamics. We disassemble clusters ranging in size from 5 to 27 argon-like
Lennard-Jones atoms, thermalised at 60 K, and obtain excess free energies, by
means of the Jarzynski equality, that are consistent with previous studies. We
only simulate the cluster of interest, in contrast to approaches that require a
series of comparisons to be made between clusters differing in size by one
molecule. We discuss the advantages and disadvantages of the scheme and how it
might be applied to more complex systems.
