  We analyze the nature of the structural order established in liquid TIP4P
water in the framework provided by the multi-particle correlation expansion of
the statistical entropy. Different regimes are mapped onto the phase diagram of
the model upon resolving the pair entropy into its translational and
orientational components. These parameters are used to quantify the relative
amounts of positional and angular order in a given thermodynamic state, thus
allowing a structurally unbiased definition of low-density and high-density
water. As a result, the structurally anomalous region within which both types
of order are simultaneously disrupted by an increase of pressure at constant
temperature is clearly identified through extensive molecular-dynamics
simulations.
