  We summarize recent numerical results on the control of the star formation
efficiency (SFE), addressing the effects of turbulence and the magnetic field
strength. In closed-box numerical simulations, the effect of the turbulent Mach
number $\Ms$ depends on whether the turbulence is driven or decaying: In driven
regimes, increasing $\Ms$ decreases the SFE, while in decaying regimes the
converse is true. The efficiencies in non-magnetic cases for realistic Mach
numbers $\Ms \sim 10$ are somewhat too high compared to observed values.
Including the magnetic field can bring the SFE down to levels consistent with
observations, but the intensity of the magnetic field necessary to accomplish
this depends again on whether the turbulence is driven or decaying. In this
kind of simulations, a lifetime of the molecular cloud (MC) needs to be
assumed. Further progress requires determining the true nature of the
turbulence driving and the lifetimes of the clouds. Simulations of MC formation
by large-scale compressions in the warm neutral medium (WNM) show that the
clouds' initial turbulence is produced by the accumulation process that forms
them, and that the turbulence is driven for as long as this process lasts,
producing realistic velocity dispersions and also thermal pressures in excess
of the mean WNM value. In simulations including self-gravity, but neglecting
the magnetic field and stellar energy feedback, the clouds never reach an
equilibrium state, but rather evolve secularly, increasing their mass and
gravitational energy until they engage in generalized gravitational collapse.
However, local collapse events begin midways through this process, and produce
enough stellar objetcs to disperse the cloud or at least halt its collapse
before the latter is completed.
