The physics and modes of star cluster formation: simulations
  We review progress in numerical simulations of star cluster formation. These
simulations involve the bottom-up assembly of clusters through hierarchical
mergers, which produces a fractal stellar distribution at young (~0.5 Myr)
ages. The resulting clusters are predicted to be mildly aspherical and highly
mass-segregated, except in the immediate aftermath of mergers. The upper
initial mass function within individual clusters is generally somewhat flatter
than for the aggregate population. Recent work has begun to clarify the factors
that control the mean stellar mass in a star-forming cloud and also the
efficiency of star formation. The former is sensitive to the thermal properties
of the gas while the latter depends both on the magnetic field and the initial
degree of gravitational boundedness of the natal cloud. Unmagnetized clouds
that are initially bound undergo rapid collapse, which is difficult to reverse
by ionization feedback or stellar winds.
