A set of Smoothed Particle Hydrodynamics simulations of the influence of photoionising radiation and stellar winds on a series of 10 ^ { 4 } M _ { \odot } turbulent molecular clouds with initial virial ratios of 0.7 , 1.1 , 1.5 , 1.9 and 2.3 and initial mean densities of 136 , 1135 and 9096 cm ^ { -3 } are presented .
Reductions in star formation efficiency rates are found to be modest , in the range 30 \% - 50 \% and to not vary greatly across the parameter space .
In no case was star formation entirely terminated over the \approx 3 Myr duration of the simulations .
The fractions of material unbound by feedback are in the range 20 - 60 \% , clouds with the lowest escape velocities being the most strongly affected . Leakage of ionised gas leads to the HII regions rapidly becoming underpressured .
The destructive effects of ionisation are thus largely not due to thermally–driven expansion of the HII regions , but to momentum transfer by photoevaporation of fresh material .
Our simulations have similar global ionisation rates and we show that the effects of feedback upon them can be adequately modelled as a steady injection of momentum , resembling a momentum–conserving wind .