Richard C. Remsing

Accurate first-principles structures and energies of diversely bonded systems from an efficient density functional

One atom or molecule binds to another through various types of bond, the strengths of which range from several meV to several eV. Although some computational methods can provide accurate descriptions of all bond types, those methods are not efficient enough for many studies (for example, large systems, ab initio molecular dynamics and high-throughput searches for functional materials). Here, we show that the recently developed non-empirical strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation (meta-GGA) within the density functional theory framework predicts accurate geometries and energies of diversely bonded molecules and materials (including covalent, metallic, ionic, hydrogen and van der Waals bonds). This represents a significant improvement at comparable efficiency over its predecessors, the GGAs that currently dominate materials computation. Often, SCAN matches or improves on the accuracy of a computationally expensive hybrid functional, at almost-GGA cost. SCAN is therefore expected to have a broad impact on chemistry and materials science.

Nickel Confined in the Interlayer Region of Birnessite: an Active Electrocatalyst for Water Oxidation

We report a synthetic method to enhance the electrocatalytic activity of birnessite for the oxygen evolution reaction (OER) by intercalating Ni(2+) ions into the interlayer region. Electrocatalytic studies showed that nickel (7.7 atomicu2009%)-intercalated birnessite exhibits an overpotential (η) of 400u2005mV for OER at an anodic current of 10u2005mAu2009cm(-2) . This η is significantly lower than the η values for birnessite (η≈700u2005mV) and the active OER catalyst β-Ni(OH)2 (η≈550u2005mV). Molecular dynamics simulations suggest that a competition among the interactions between the nickel cation, water, and birnessite promote redox chemistry in the spatially confined interlayer region.

Ab initio theory and modeling of water

Significance Water is vital to our everyday life, but its structure at a molecular level is still not fully understood from either experiment or theory. The latter is hampered by our inability to construct a purely predictive, first principles model. The difficulty in modeling water lies in capturing the delicate interplay among the many strong and weak forces that govern its behavior and phase diagram. Herein, molecular simulations with a recently proposed nonempirical quantum mechanical approach (the SCAN density functional) yield an excellent description of the structural, electronic, and dynamic properties of liquid water. SCAN (strongly constrained and appropriately normed)-based approaches, which describe diverse types of bonds in materials on an equal, accurate footing, will likely enable efficient and reliable modeling of aqueous phase chemistry. 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.

A Free‐Standing Molecular Spin–Charge Converter for Ubiquitous Magnetic‐Energy Harvesting and Sensing

Magnetic-energy harvesting in a centimeter-sized free-standing (BEDT-TTF)C60 charge-transfer single crystal is demonstrated. The crystal shows sensitive magnetic-, thermal-, and mechanical-sensing ability, with an excellent piezoresistance coefficient of -5.1 × 10-6 Pa-1 . The self-powered sensing performance, together with its solution processability and flexibility, endow it with the capability of driving a new generation of noncontact magnetic-energy harvesting and sensing technologies.

Long-ranged contributions to solvation free energies from theory and short-ranged models.

Significance Many important biological and industrial processes, ranging from protein folding and ligand binding to self-assembly of materials at interfaces, take place in solution and are mediated by driving forces rooted in solvation. However, conceptual and computational difficulties arising from long-ranged Coulomb interactions still present a challenge to current approaches. Here we present a framework to determine very accurately the long-ranged contributions to solvation free energies in charged and polar systems from models with only short-ranged, local interactions. We examine a variety of ubiquitous solvation processes, including hydrophobic and ionic hydration, as well as colloidal overcharging. The theory additionally suggests ways to improve density functional theories of solvation by providing insights into commonly used approximations. Long-standing problems associated with long-ranged electrostatic interactions have plagued theory and simulation alike. Traditional lattice sum (Ewald-like) treatments of Coulomb interactions add significant overhead to computer simulations and can produce artifacts from spurious interactions between simulation cell images. These subtle issues become particularly apparent when estimating thermodynamic quantities, such as free energies of solvation in charged and polar systems, to which long-ranged Coulomb interactions typically make a large contribution. In this paper, we develop a framework for determining very accurate solvation free energies of systems with long-ranged interactions from models that interact with purely short-ranged potentials. Our approach is generally applicable and can be combined with existing computational and theoretical techniques for estimating solvation thermodynamics. We demonstrate the utility of our approach by examining the hydration thermodynamics of hydrophobic and ionic solutes and the solvation of a large, highly charged colloid that exhibits overcharging, a complex nonlinear electrostatic phenomenon whereby counterions from the solvent effectively overscreen and locally invert the integrated charge of the solvated object.

Water Lone Pair Delocalization in Classical and Quantum Descriptions of the Hydration of Model Ions

Understanding the nature of ionic hydration at a fundamental level has eluded scientists despite intense interest for nearly a century. In particular, the microscopic origins of the asymmetry of ion solvation thermodynamics with respect to the sign of the ionic charge remains a mystery. Here, we determine the response of accurate quantum mechanical water models to strong nanoscale solvation forces arising from excluded volumes and ionic electrostatic fields. This is compared to the predictions of two important limiting classes of classical models of water with fixed point changes, differing in their treatment of lone pair electrons. Using the quantum water model as our standard of accuracy, we find that a single fixed classical treatment of lone pair electrons cannot accurately describe solvation of both apolar and cationic solutes, emphasizing the need for a more flexible description of local electronic effects in solvation processes. However, we explicitly show that all water models studied respond to weak long-ranged electrostatic perturbations in a manner that follows macroscopic dielectric continuum models, as would be expected. We emphasize the importance of these findings in the context of realistic ion models, using density functional theory and empirical models, and discuss the implications of our results for quantitatively accurate reduced descriptions of solvation in dielectric media.

Capillary evaporation of the ionic liquid [EMIM][BF4] in nanoscale solvophobic confinement

Solvent density fluctuations play a crucial role in liquid-vapor transitions in solvophobic confinement and can also be important for understanding solvation of polar and apolar solutes. In the case of ionic liquids (ILs), density fluctuations can be used to understand important processes in the context of nanoscale aggregation and colloidal self-assemblies. In this article, we explore the nature of density fluctuations associated with capillary evaporation of the IL 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) in the confined region of model solvophobic nanoscale sheets by using molecular dynamics simulations combined with non-Boltzmann sampling techniques. We demonstrate that density fluctuations of the confined IL play an important role in capillary evaporation, suggesting analogies to dewetting transitions involving water. Significant changes in the interfacial structure of the IL are also detailed and suggested to underlie a non-classical (non-parabolic) dependence of the free energy barrier to evaporation on the degree of confinement.

Light-induced dilation in nanosheets of charge-transfer complexes

Significance Photostrictive compounds are promising device materials because of their fundamental photophysical properties and light-induced strain applications. Although molecular charge-transfer crystals exhibiting light–matter interactions have been successfully deployed in optoelectronics, an air-stable molecular material that couples photons and electrons, achieving photostriction through the coupling of light and mechanical degrees of freedom, has not yet been discovered. Herein, we report a substantial light-induced dilation in a molecular material at room temperature, which is accompanied by simultaneous photocurrent generation. This finding opens avenues for coupling optical, electronic, and mechanical functionalities for possible use in remote wireless photoswitchable devices. We report the observation of a sizable photostrictive effect of 5.7% with fast, submillisecond response times, arising from a light-induced lattice dilation of a molecular nanosheet, composed of the molecular charge-transfer compound dibenzotetrathiafulvalene (DBTTF) and C60. An interfacial self-assembly approach is introduced for the thickness-controlled growth of the thin films. From photoabsorption measurements, molecular simulations, and electronic structure calculations, we suggest that photostriction within these films arises from a transformation in the molecular structure of constituent molecules upon photoinduced charge transfer, as well as the accommodation of free charge carriers within the material. Additionally, we find that the photostrictive properties of the nanosheets are thickness-dependent, a phenomenon that we suggest arises from surface-induced conformational disorder in the molecular components of the film. Moreover, because of the molecular structure in the films, which results largely from interactions between the constituent π-systems and the sulfur atoms of DBTTF, the optoelectronic properties are found to be anisotropic. This work enables the fabrication of 2D molecular charge-transfer nanosheets with tunable thicknesses and properties, suitable for a wide range of applications in flexible electronic technologies.

Bonding in the metallic molecular solid α-Gallium

ABSTRACT Solid, liquid and alloyed phases of gallium play a role in a variety of important technological applications. While many of the gallium phases involved in these applications are metallic, some have been proposed or are known to contain covalently bound Ga dimers. Thus, understanding the nature of bonding in Ga is crucial to the development of Ga-based materials. The solid phase of gallium at ambient conditions, α-Ga, is metallic and composed of molecular dimers, and can serve as a testing ground for studying gallium bonding with electronic structure calculations. We use density functional theory-based molecular dynamics simulations in conjunction with maximally localised Wannier functions to examine the nature of chemical bonding in α-Ga. We propose a geometric criterion for defining various bonding environments, which enables the quantification of covalent and weak bonds in solid gallium. We additionally connect the bonding structure of α-Ga to its phonon density of states and discuss similarities and differences with diatomic halogen crystals. GRAPHICAL ABSTRACT