Computational Methods for Molecular Dynamics of Supercooled Water Between 200 and 273 K.

Water exhibits anomalous thermodynamic and structural behaviour as it approaches and crosses below its melting point. Modelling this behaviour computationally remains challenging: as temperature decreases, nuclear quantum effects (NQEs) become increasingly significant, sampling efficiency deteriorates, and the choice of computational method critically impacts the accuracy of predicted structural and dynamical properties. This review aims to provide practitioners with a practical guide to performing reliable molecular dynamics simulations of water at low temperatures, with emphasis on the supercooled regime (200-273 K). We examine commonly used ab initio molecular dynamics (AIMD) methods and critically evaluate strategies for incorporating nuclear quantum effects through path-integral molecular dynamics (PIMD), ring-polymer MD (RPMD), and centroid MD (CMD). On cooling through the supercooled regime, the relative importance of nuclear quantum effects increases steadily, so that the error incurred by treating the nuclei classically grows and affects the structural and dynamical properties of interest. The known limitations of density functional approximations for water's structure are assessed in the context of their interplay with NQE treatment, rather than in isolation. By discussing practical considerations, including sampling efficiency and the temperature scaling of path-integral bead counts, alongside the methods themselves, this review is intended to serve as a reference for reliable AIMD studies of supercooled water.