Quantum ESPRESSO is an integrated suite of open-source computer codes for quantum simulations of materials using state-of-the art electronic-structure techniques, based on density-functional theory, density-functional perturbation theory, and many-body perturbation theory, within the plane-wave pseudo-potential and projector-augmented-wave approaches. Quantum ESPRESSO owes its popularity to the wide variety of properties and processes it allows to simulate, to its performance on an increasingly broad array of hardware architectures, and to a community of researchers that rely on its capabilities as a core open-source development platform to implement theirs ideas. In this paper we describe recent extensions and improvements, covering new methodologies and property calculators, improved parallelization, code modularization, and extended interoperability both within the distribution and with external software.

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...

In this work, we report the results of a series of density functional theory (DFT) based ab initio molecular dynamics (AIMD) simulations of ambient liquid water using a hierarchy of exchange-correlation (XC) functionals to investigate the individual and collective effects of exact exchange (Exx), via the PBE0 hybrid functional, non-local vdW/dispersion interactions, via a fully self-consistent density-dependent dispersion correction, and approximate nuclear quantum effects (aNQE), via a 30 K increase in the simulation temperature, on the microscopic structure of liquid water. Based on these AIMD simulations, we found that the collective inclusion of Exx, vdW, and aNQE as resulting from a large-scale AIMD simulation of (H2O)128 at the PBE0+vdW level of theory, significantly softens the structure of ambient liquid water and yields an oxygen-oxygen structure factor, SOO(Q), and corresponding oxygen-oxygen radial distribution function, gOO(r), that are now in quantitative agreement with the best available experimental data. This level of agreement between simulation and experiment as demonstrated herein originates from an increase in the relative population of water molecules in the interstitial region between the first and second coordination shells, a collective reorganization in the liquid phase which is facilitated by a weakening of the hydrogen bond strength by the use of the PBE0 hybrid XC functional, coupled with a relative stabilization of the resultant disordered liquid water configurations by the inclusion of nonlocal vdW/dispersion interactions. This increasingly more accurate description of the underlying hydrogen bond network in liquid water obtained at the PBE0+vdW+aNQE level of theory yields higher-order correlation functions, such as the oxygen-oxygen-oxygen triplet angular distribution, POOO(θ), and therefore the degree of local tetrahedrality, as well as electrostatic properties, such as the effective molecular dipole moment, that are also in much better agreement with experiment.

The adsorption of aromatic molecules on metal surfaces plays a key role in condensed matter physics and functional materials. Depending on the strength of the interaction between the molecule and the surface, the binding is typically classified as either physisorption or chemisorption. Van der Waals (vdW) interactions contribute significantly to the binding in physisorbed systems, but the role of the vdW energy in chemisorbed systems remains unclear. Here we study the interaction of benzene with the (111) surface of transition metals, ranging from weak adsorption (Ag and Au) to strong adsorption (Pt, Pd, Ir, and Rh). When vdW interactions are accurately accounted for, the barrier to adsorption predicted by standard density-functional theory (DFT) calculations essentially vanishes, producing a metastable precursor state on Pt and Ir surfaces. Notably, vdW forces contribute more to the binding of covalently bonded benzene than they do when benzene is physisorbed.Comparison to experimental data demonstrates that some of the recently developed methods for including vdW interactions in DFT allow quantitative treatment of both weakly and strongly adsorbed aromatic molecules on metal surfaces, extending the already excellent performance found for molecules in the gas phase.