Water is of the utmost importance for life and technology. However, a genuinely predictive ab initio model of water has eluded scientists. We demonstrate that a fully ab initio approach, relying on the strongly constrained and appropriately normed (SCAN) density functional, provides such a description of water. SCAN accurately describes the balance among covalent bonds, hydrogen bonds, and van der Waals interactions that dictates the structure and dynamics of liquid water. Notably, SCAN captures the density difference between water and ice Ih at ambient conditions, as well as many important structural, electronic, and dynamic properties of liquid water. These successful predictions of the versatile SCAN functional open the gates to study complex processes in aqueous phase chemistry and the interactions of water with other materials in an efficient, accurate, and predictive, ab initio manner.water | ab initio theory | hydrogen bonding | density functional theory | molecular dynamics W ater is arguably the most important molecule for life and is involved in almost all biological processes. Without water, life, as we know it, would not exist, earning water the pseudonym "matrix of life," among others (1). Despite the apparent simplicity of an H2O molecule, water in the condensed phase displays a variety of anomalous properties that originate from its complex structure. In an ideal arrangement, water molecules form a tetrahedral network of hydrogen (H) bonds with each vertex being occupied by a water molecule. This tetrahedral network is realized in the solid phase ice Ih, but thermal fluctuations disrupt the H-bond network in the liquid state, with the network fluctuating on picosecond to nanosecond timescales. Due to the complexity of the H-bond network and its competition with thermal fluctuations, a precise molecular-level understanding of the structure of liquid water remains elusive. Major challenges lie in unambiguously capturing the atomic-scale fluctuations in water experimentally. Current approaches such as time-resolved spectroscopy (2, 3) and diffraction measurements (4, 5) may be able to resolve changes on picosecond timescales but rely on interpretation through models, which often cannot describe all of the details of liquid water with quantitative accuracy. Not surprisingly, the nature of the H-bond network in liquid water continues to be at the center of scientific debate, and advances in both experiment and theory are needed, especially with regard to quantitative modeling of aqueous phase chemistry.Ab initio molecular dynamics (AIMD) simulation (6) is an ideal approach for modeling the condensed phases of water across the phase diagram and aqueous phase chemistry using quantum mechanical principles (7-11), although for some applications, such as the study of liquid vapor phase equilibria (12), Monte Carlo methods are better suited. In particular, Kohn-Sham density functional theory (DFT) (13)-used to model the system in its electronic ground state-provides an efficient framework that enables the si...

Proton transfer via hydronium and hydroxide ions in water is ubiquitous. It underlies acid-base chemistry, certain enzyme reactions, and even infection by the flu. Despite two centuries of investigation, the mechanism underlying why hydroxide diffuses slower than hydronium in water is still not well understood. Herein, we employ state-of-the-art density-functional-theory-based molecular dynamics-with corrections for non-local van der Waals interactions, and self-interaction in the electronic ground state-to model water and hydrated water ions. At this level of theory, we show that structural diffusion of hydronium preserves the previously recognized concerted behaviour. However, by contrast, proton transfer via hydroxide is less temporally correlated, due to a stabilized hypercoordination solvation structure that discourages proton transfer. Specifically, the latter exhibits non-planar geometry, which agrees with neutron-scattering results. Asymmetry in the temporal correlation of proton transfer leads to hydroxide diffusing slower than hydronium.

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We describe how to apply the recently developed pole expansion and selected inversion (PEXSI) technique to Kohn-Sham density function theory (DFT) electronic structure calculations that are based on atomic orbital discretization. We give analytic expressions for evaluating the charge density, the total energy, the Helmholtz free energy and the atomic forces (including both the Hellman-Feynman force and the Pulay force) without using the eigenvalues and eigenvectors of the Kohn-Sham Hamiltonian. We also show how to update the chemical potential without using Kohn-Sham eigenvalues. The advantage of using PEXSI is that it has a much lower computational complexity than that associated with the matrix diagonalization procedure. We demonstrate the performance gain by comparing the timing of PEXSI with that of diagonalization on insulating and metallic nanotubes. For these quasi-1D systems, the complexity of PEXSI is linear with respect to the number of atoms. This linear scaling can be observed in our computational experiments when the number of atoms in a nanotube is larger than a few hundreds. Both the wall clock time and the memory requirement of PEXSI is modest. This makes it even possible to perform Kohn-Sham DFT calculations for 10,000-atom nanotubes with a sequential implementation of the selected inversion algorithm. We also perform an accurate geometry optimization calculation on a truncated (8,0) boron-nitride nanotube system containing 1024 atoms. Numerical results indicate that the use of PEXSI does not lead to loss of accuracy required in a practical DFT calculation.

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