region inside the first hydrostatic core was modelled and the initial conditions were chosen somewhat artificially. The ability to perform three-dimensional calculations which follow the collapse of molecular cloud cores to stellar densities now allows us to study the formation of close binaries from the collapse of larger-scale ($\approx 10000$ AU) molecular cloud cores.

The evolution of an accreting protobinary system {#accretion}
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The favoured mechanism for the formation of most binary stellar systems is the fragmentation of a collapsing molecular cloud core. Fragmentation has been studied numerically for $\approx 20$ years. These calculations appear to show that it is possible to form binaries with similar properties to those that are observed via fragmentation. However, they have not allowed us to predict the fundamental properties of stellar systems such as the fraction of stellar systems which are binary or the properties of binary systems (e.g. the distributions of mass ratios, separations, and eccentricities and the properties of discs in pre-main-sequence systems).

There are two primary reasons for this lack of predictive power. First, the results of fragmentation calculations depend sensitively on the initial conditions, which are poorly constrained. The second problem is that of accretion. Fragmentation calculations are typically stopped 