Data from "Melting curves of ice polymorphs in the vicinity of the liquid-liquid critical point"

The possible existence of a liquid-liquid critical point in deeply
supercooled water has been a subject of debate in part due to the
challenges associated with providing definitive experimental evidence.
Pioneering work by Mishima and Stanley [Nature 392, 164 (1998) and
Phys.~Rev.~Lett. 85, 334 (2000)] sought to shed light on this problem by
studying the melting curves of different ice polymorphs and their
metastable continuation in the vicinity of the expected location of the
liquid-liquid transition and its associated critical point. Based on the
continuous or discontinuous changes in slope of the melting curves,
Mishima suggested that the liquid-liquid critical point lies between the
melting curves of ice III and ice V. Here, we explore this conjecture
using molecular dynamics simulations with a purely-predictive machine
learning model based on ab initio quantum-mechanical calculations. We
study the melting curves of ices III, IV, V, VI, and XIII using this model
and find that the melting lines of all the studied ice polymorphs are
supercritical and do not intersect the liquid-liquid transition locus. We
also find a pronounced, yet continuous, change in slope of the melting
lines upon crossing of the locus of maximum compressibility of the liquid.
Finally, we analyze critically the literature in light of our findings,
and conclude that the scenario in which melting curves are supercritical
is favored by the most recent computational and experimental evidence.
Thus, although the preponderance of experimental and computational
evidence is consistent with the existence of a second critical point in
water, the behavior of the melting lines of ice polymorphs does not
provide strong evidence in support of this viewpoint, according to our
calculations.