Molecular-scale remnants of the liquid-gas transition in supercritical polar fluids

An electronically coarse-grained model for water reveals a persistent vestige of the liquid-gas transition deep into the supercritical region. A crossover in the density dependence of the molecular dipole arises from the onset of non-percolating hydrogen bonds. The crossover points coincide with the Widom line in the scaling region but extend further, tracking the heat capacity maxima, offering evidence for liquid- and gas-like state points in a "one-phase" fluid. The effect is present even in dipole-limit models suggesting that it is common for all molecular liquids exhibiting dipole enhancement in the liquid phase.