  We present a new set of analytic models for the expansion of HII regions
powered by UV photoionisation from massive stars and compare them to a new
suite of radiative magnetohydrodynamic simulations of turbulent,
self-gravitating molecular clouds. To perform these simulations we use the
Eulerian adaptive mesh magnetohydrodynamics code RAMSES-RT, including radiative
transfer of UV photons. Our analytic models successfully predict the global
behaviour of the HII region provided the density and velocity structure of the
cloud is known. We give estimates for the HII region behaviour based on a power
law fit to the density field assuming that the system is virialised. We give a
radius at which the ionisation front should stop expanding ("stall"). If this
radius is smaller than the distance to the edge of the cloud, the HII region
will be trapped by the cloud. This effect is more severe in collapsing clouds
than in virialised clouds, since the density in the former increases
dramatically over time, with much larger photon emission rates needed for the
HII region to escape a collapsing cloud. We also measure the response of Jeans
unstable gas to the HII regions to predict the impact of UV radiation on star
formation in the cloud. We find that the mass in unstable gas can be explained
by a model in which the clouds are evaporated by UV photons, suggesting that
the net feedback on star formation should be negative
