  We examine the effects of gas expulsion on initially sub-structured and
out-of-equilibrium star clusters. We perform $N$-body simulations of the
evolution of star clusters in a static background potential before removing
that potential to model gas expulsion. We find that the initial star formation
efficiency is not a good measure of the survivability of star clusters. This is
because the stellar distribution can change significantly, causing a large
change in the relative importance of the stellar and gas potentials. We find
that the initial stellar distribution and velocity dispersion are far more
important parameters than the initial star formation efficiency, and that
clusters with very low star formation efficiencies can survive gas expulsion.
We suggest that it is variations in cluster initial conditions rather than in
their star formation efficiencies that cause some clusters to be destroyed
while a few survive.
