  We investigate the variation of the driving force for crystallization of a
supercooled liquid along isomorphs, curves along which structure and dynamics
are invariant. The variation is weak, and can be predicted accurately for the
Lennard-Jones fluid using a recently developed formalism and data at a
reference temperature. More general analysis allows interpretation of
experimental data for molecular liquids such as dimethyl phthalate and
indomethacin, and suggests that the isomorph scaling exponent $\gamma$ in these
cases is an increasing function of density, although this cannot be seen in
measurements of viscosity or relaxation time.
