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...

No abstract

We perform ab initio molecular dynamics (AIMD) simulation of liquid water in the canonical ensemble at ambient conditions using the strongly constrained and appropriately normed (SCAN) meta-generalized-gradient approximation (GGA) functional approximation and carry out systematic comparisons with the results obtained from the GGA-level Perdew-Burke-Ernzerhof (PBE) functional and Tkatchenko-Scheffler van der Waals (vdW) dispersion correction inclusive PBE functional. We analyze various properties of liquid water including radial distribution functions, oxygen-oxygen-oxygen triplet angular distribution, tetrahedrality, hydrogen bonds, diffusion coefficients, ring statistics, density of states, band gaps, and dipole moments. We find that the SCAN functional is generally more accurate than the other two functionals for liquid water by not only capturing the intermediate-range vdW interactions but also mitigating the overly strong hydrogen bonds prescribed in PBE simulations. We also compare the results of SCAN-based AIMD simulations in the canonical and isothermal-isobaric ensembles. Our results suggest that SCAN provides a reliable description for most structural, electronic, and dynamical properties in liquid water.

No abstract

Electron-hole excitation theory is used to unveil the role of nuclear quantum effects on the X-ray absorption spectral signatures of water, whose structure is computed via path-integral molecular dynamics with the MB-pol intermolecular potential model. Compared to spectra generated from the classically modeled water, quantum nuclei introduce important effects on the spectra in terms of both the energies and line shapes. Fluctuations due to delocalized protons influence the short-range ordering of the hydrogen bond network via changes in the intramolecular covalence, which broaden the pre-edge spectra. For intermediate-range and long-range ordering, quantum nuclei approach the neighboring oxygen atoms more closely than classical protons, promoting an "ice-like" spectral feature with the intensities shifted from the main-to post-edge. Computed spectra are in nearly quantitative agreement with the available experimental data.

Manipulation of an isolated water nanodroplet (WN) on certain surfaces is important to various nanofluidic applications but challenging. Here we present a digital nanofluidic system based on a graphene/water/mica sandwich structure. In this architecture, graphene provides a flexible protection layer to isolate WNs from the outside environment, and a monolayer ice-like layer formed on the mica surface acts as a lubricant layer to allow these trapped WNs to move on it freely. In combination with scanning probe microscope techniques, we are able to move, merge, and separate individual water nanodroplets in a controlled manner. The smallest manipulatable water nanodroplet has a volume down to yoctoliter (10(-24) L) scale.

No abstract

For the interlayer dielectric in microelectronics, light element compounds are preferably accepted due to less electronic polarization. Here, the nontrivial dielectric nature of the Sb 4 O 6 cage-molecular crystal, known as α-antimony trioxide (α-Sb 2 O 3 ), is reported. The gas-phase synthesized α-Sb 2 O 3 nanoflakes are of high crystal quality, from which the abnormal local admittance responses were revealed by scanning microwave impedance microscopy (sMIM). The remarkably low dielectric constant (k), 2.0∼2.5, is corroborated by the analysis of the thickness-dependent sMIM-capacitance signal. In light of the theoretical calculations, the ultralow molecular density and the significantly suppressed ionic polarization are both crucial to the highly reduced k. Combining with the excellent optical band gap, thermal stability, and breakdown strength, α-Sb 2 O 3 is a promising low k dielectric. KEYWORDS: low k, α-Sb 2 O 3 , Sb 4 O 6 cage, molecular crystal, interlayer dielectric