  We review the available experimental information on the thermodynamic
properties of supercooled ordinary and heavy water and demonstrate the
possibility of modeling these thermodynamic properties on a theoretical basis.
We show that by assuming the existence of a liquid-liquid critical point in
supercooled water, the theory of critical phenomena can give an accurate
account of the experimental thermodynamic-property data up to a pressure of 150
MPa. In addition, we show that a phenomenological extension of the theoretical
model can account for all currently available experimental data in the
supercooled region, up to 400 MPa. The stability limit of the liquid state and
possible coupling between crystallization and liquid-liquid separation are
discussed. It is concluded that critical-point thermodynamics describes the
available thermodynamic data for supercooled water within experimental
accuracy, thus establishing a benchmark for further developments in this area.
