  Thermodynamic properties of the ST2 model for supercooled liquid water
exhibit anomalies similar to those observed in real water. A possible
explanation of these anomalies is the existence of a metastable, liquid-liquid
transition terminated by a critical point. This phenomenon, whose possible
existence in real water is the subject of much current experimental work, has
been unambiguously demonstrated for this particular model by most recent
simulations. In this work, we reproduce the anomalies of two versions of the
ST2 model with an equation of state describing water as a non-ideal "mixture"
of two different types of local molecular order. We show that the liquid-liquid
transition in the ST2 water is energy-driven. This is in contrast to another
popular model, mW, in which non-ideality in mixing of two alternative local
molecular orders is entropy-driven, and is not sufficiently strong to induce a
liquid-liquid transition.
