  Test area deformations are used to analyse vapour-liquid interfaces of
Lennard-Jones particles by molecular dynamics simulation. For planar
vapour-liquid interfaces the change in free energy is captured by the average
of the corresponding change in energy, the leading-order contribution. This is
consistent with the commonly used mechanical (pressure tensor) route for the
surface tension. By contrast for liquid drops one finds a large second-order
contribution associated with fluctuations in energy. Both the first- and
second-order terms make comparable contributions, invalidating the mechanical
relation for the surface tension of small drops. The latter is seen to increase
above the planar value for drop radii of ~8 particle diameters, followed by an
apparent weak maximum and slow decay to the planar limit, consistent with a
small negative Tolman length.
