  We study both analytically and numerically the disruptive effect of
instantaneous gas removal from an embedded cluster. We setup a calculation
based on the stellar velocity distribution function, to compute the fraction of
stars that remain bound once the cluster has ejected the gas and is out of
equilibrium. We find tracks of bound mass-fraction vs star formation efficiency
similar to those obtained with N-body calculations. We use these to argue that
embedded clusters must develop high-binding energy cores if they are to survive
as bound clusters despite a star formation rate as low as 20 per cent or lower
suggested by observations.
