Figure 4. Supercooled water.

(a) An orthographic projection of the reversible free-energy surface described by density and the crystalline order parameter, Q₆, for ST2 water at a point of liquid–liquid coexistence (228.6 K and 2.4 kbar). (Adapted from ref. 20). (b) Low-density fraction from the predictions from the cross-over equation of state extrapolated from the ST2 model as a function of temperature at various pressures. (Adapted from ref. 70). (c) A train of droplets flows in vacuum perpendicular to ∼50 fs long X-ray pulses. A coherent scattering pattern from a water droplet was recorded when a single droplet was in the interaction region at the time of arrival of a single X-ray pulse. Each diffraction pattern is classified either as a water shot exclusively containing pure liquid scattering characterized by a diffuse water ring or as an ice shot characterized by intense and discrete Bragg peaks superposed on the water-scattering ring. (Adapted from ref. 71). (d) Magnitude of the second g(r) peak, g₂, as function of temperature for SPC/E and TIP4P/2005 simulations, hypothetical LLT and a curve fitted to the experimental data71 but shifted to 5 K lower temperatures to cross the TIP4P/2005 simulation curve at its Widom line. LDA marks g₂ for low-density amorphous ice.