  Because of the isotropic and disordered nature of liquids, the anisotropy
hidden in intermolecular interactions are often neglected. Accordingly, the
order parameter describing a simple liquid has so far been believed to be only
density. In contrast to this common sense, we propose that two order
parameters, namely, density and bond order parameters, are required to describe
the phase behavior of liquids since they intrinsically tend to form local
bonds. This model gives us clear physical explanations for two
poorly-understood phenomena in supercooled liquids: (i) large-scale density
fluctuations and (ii) phase separation of a one-component liquid into two
liquid phases.
