Supercritical Water at Near-Experimental Accuracy with MB-pol(2023)

Supercritical water (SCW) exhibits anomalous thermophysical properties that are notoriously challenging to reproduce computationally due to the progressive disruption of the hydrogen-bond network and the interplay between many-body interactions, density fluctuations, and thermal disorder. Here, we demonstrate that the data-driven many-body MB-pol(2023) potential quantitatively reproduces the liquid-to-supercritical crossover of water with near-experimental accuracy over temperatures between 400 and 900 K and pressures from 25 to 100 MPa. Molecular dynamics simulations show that MB-pol(2023) accurately captures the temperature evolution of density and reproduces the anomalous behavior of the isothermal compressibility, thermal expansivity, and heat capacity near the critical region. Along the 25 MPa isobar, these response functions exhibit pronounced maxima between 650 and 670 K, yielding a pseudo-critical Widom-line temperature of TWL ≈ 660 ± 4 K, in excellent agreement with experimental estimates from NIST and IAPWS data (i.e., 660 K). Structural analysis reveals a continuous collapse of tetrahedral order and hydrogen-bond connectivity across the crossover, accompanied by the disappearance of medium-range correlations in radial distribution functions. The hydrogen-bond network evolution correlates almost universally with density, independently of pressure, supporting the interpretation of SCW as a density-driven fluid. Dynamical observables further reveal a sharp increase in molecular mobility near the Widom line. Complementary ab initio molecular dynamics simulations at 900 K and 25 MPa show no evidence of water dissociation and confirm the reliability of MB-pol(2023) under these extreme conditions. Although trained exclusively on accurate quantum-mechanical data without explicit thermodynamic information, MB-pol(2023) emerges as a quantitatively reliable framework for describing water across the liquid-to-supercritical continuum.