  With computer simulations of the hard sphere model, we examine in detail the
microscopic pathway connecting the metastable melt to the emergence of
crystalline clusters. In particular we will show that the nucleation of the
solid phase does not follow a two-step mechanism, where crystals form inside
dense precursor regions. On the contrary, we will show that nucleation is
driven by fluctuations of orientational order, and not by the density
fluctuations. By considering the development of the pair-excess entropy inside
crystalline nuclei, we confirm that orientational order precedes positional
order. These results are at odd with the idea of a two-step nucleation
mechanism for fluids without a metastable liquid-liquid phase separation. Our
study suggests the pivotal role of bond orientational ordering in triggering
crystal nucleation.
