Local Structure in~\textit{Ab Initio} Liquid Water: Signatures of Amorphous Phases

Within the framework of density functional theory, the inclusion of exact exchange and non-local van der Waals/dispersion interactions is crucial for predicting a microscopic structure of ambient liquid water that quantitatively agrees with experiment [1]. In this work, we have used the local structure index (LSI) order parameter to analyze the local structure in such highly accurate \textit{ab initio} liquid water. At ambient conditions, the LSI probability distribution, P($I$), was unimodal with most water molecules characterized by more disordered high-density-like local environments. With thermal excitations removed, the resultant bimodal P($I$) in the inherent potential energy surface (IPES) exhibited a 3:1 ratio between high- and low-density-like molecules, with the latter forming small connected clusters amid the predominant population. By considering the spatial correlations and hydrogen bond network topologies \textit{among} water molecules with the same LSI identities, we demonstrate that the signatures of the experimentally observed low- and high-density amorphous phases of ice are present in the IPES of ambient liquid water [2]. [1] DiStasio \emph{et al.}, J. Chem. Phys. \textbf{141}, 084502 (2014). [2] Santra \emph{et al.}, Mol. Phys. \textbf{113}, 2829 (2015).