Nuclear Quantum Effects Reshape Structural Signatures of Supercooled Water near the Liquid-Liquid Critical Region.

The liquid-liquid transition scenario for supercooled water relies heavily on structural markers that distinguish low-density-liquid-like from high-density-liquid-like environments. Because these markers are often evaluated with classical nuclei, it remains unclear how zero-point motion changes the interpretation of the putative liquid-liquid critical region. Here, we compare classical molecular dynamics (MD) and path-integral molecular dynamics (PIMD) simulations of a flexible q-TIP4P/F-like water model over temperatures and pressures spanning the thermodynamic region where this model is expected to exhibit a liquid-liquid critical point. Classical trajectories display a sharp density increase at 180 K between 180 and 220 MPa, whereas path-integral simulations give a smoother pressure response. Nuclear quantum effects (NQE) broaden oxygen-oxygen, oxygen-hydrogen, and hydrogen-hydrogen correlations and reduce the first-shell tetrahedral order, yet they slightly increase the nearest-neighbor Steinhardt Q6 parameter. Thus, quantum nuclei do not simply blur all structural signatures uniformly; they renormalize different order parameters in different directions. These results identify nuclear quantum motion as a necessary ingredient when assigning LDL-like and HDL-like structural motifs and caution against sharp two-state assignments based on static classical order parameters alone.