Comparative first-principles structural and vibrational properties of rutile and anatase TiO2.

The structural and vibrational properties of two polymorphs of TiO2, rutile and anatase, have been investigated by first-principles methods at different levels of exchange-correlational (XC) energy functionals in density functional theory (DFT). Reports in the literature to date are contradictory regarding the stability of the rutile phase using DFT XC-functionals more sophisticated than simple local-density approximation. Here the PBEsol generalized gradient approximation (GGA), TPSS meta-GGA, and HSE06 hybrid functionals have been employed to demonstrate the XC-functional effects on the calculated structural, phonon and thermodynamic properties of rutile and anatase TiO2. Lattice and elastic parameters correctly calculated with these XC-functionals show good agreement with the experimental values. Calculated phonon frequencies generated stable phonon dispersion relations for both rutile and anatase TiO2when correctly converged, in agreement with the experimental observations. The phonon frequencies along high symmetry Brillouin zone paths and their corresponding phonon density of states showed sensitivity to different levels of XC-functional employed in phonon dispersion prediction. Nevertheless, the thermodynamic properties of rutile and anatase TiO2estimated by harmonic approximations are in excellent experimental agreement and are effectively invariant to the level of theory employed in the DFT XC-functional.

Reactive Bimetallic Nanostructures Based on Triply Periodic Minimal Surfaces: A Molecular Dynamics Study toward the Limits of Performance.

A variety of intermetallic compounds possesses high enthalpies of formation. These compounds may be formed from reactive compacts or nanostructures comprised of unreacted precursor metals. These precursor structures support self-propagating high temperature synthesis (SHS) reactions which afford very high specific energy densities and rates, with excellent spatial control and a variety of useful applications. The present work compares the reactivity of notional bimetallic nanostructures based on well-known triply periodic minimal surfaces (TPMSes) with the popular reactive nanolaminate (RNL) modality for the Ni/Al system, using a molecular dynamics approach. TPMS-derived nanostructures were found to have lower ignition energies and faster reaction rates than RNLs of comparable periodicity, while the maximum achievable temperature of ignitions was found to be modulated by a complex interplay of factors including reaction rate and specific metal/metal interface density. Nanostructure reactivity and thermochemistry is also affected by effective diffusion dimensionality and recalescent precipitation of intermetallic crystallites. The TPMS-derived reactive nanostructures presented herein anticipate plausible advances in nanofabrication technology.

The Hitchhiker's Guide to the Wave Function∥.

The electronic wave function of molecules is 3N-dimensional and inseparable in the coordinates of the N electrons. Whereas molecular orbitals are often invoked to visualize the electronic structure, they are nonunique, with the same 3N-dimensional wave function being represented by an infinite number of 3-D, one-electron functions (orbitals). Furthermore, multireference wave functions cannot be described by an antisymmetrized product of a single set of occupied orbitals. What is required is a way to visualize the full dimensionality of the wave function, including the effects of correlation, as a 3N-dimensional being would be able to do. In the past 5 years, we have been developing a way to analyze and visualize highly dimensional wave functions by focusing on the structure of the repeating unit demanded by fermionic behavior. This 3N-dimensional repeating unit, the wave function "tile", can be projected onto the three dimensions of each electron, in turn, to reveal the complete electronic structure. It is found that the tile reproduces canonical chemical motifs such as core-electrons, single bonds and lone pairs. Multiple bonds emerge as the "banana" bonds favored by Pauling. As a function of the reaction coordinate, electron motions are visualized that correspond to the curly arrow notation of organic chemists. Excited states can also be inspected. Analyzing a wave function in terms of fermionic tiling allows for insight not facilitated by the inspection of orbitals or configuration interaction vectors: The wave function tiles of resonance structures reveal that electron correlation in benzene pushes opposing spin electrons to occupy alternate Kekulé structures, and in C2, the emerging structure supports the notion of a triply bonded structure with a weak, fourth bonding contribution.

The role of interlayer gases and surface asperities in compression-induced intermetallic formation in Ni/Al nanocomposites.

Reactive composites comprising alternating nano- or microscale layers of Ni and Al are known to undergo self-sustaining alloying reactions under compression loading, however the effect of infiltrated gas within the microstructure of such reactive nanolaminates-as well as the presence of asperities on the free surfaces of such composites-is not well understood. This work presents atomistic molecular dynamics simulation and analysis of the mechanical dynamics and thermal evolution of planar Ni/Al nanolaminates under a variety of scenarios of layer dimensions, surface asperity shape and orientation, and interlayer gas identity and concentration. These simulations indicate that the rate of the alloying reaction is inversely correlated with the layer width of the nanolaminate, recapitulating experimental results. The presence of surface asperities of comparable scale to the nanolaminate layer thickness enhances short-term intermetallic mixing but has a marginal accelerant effect on the reaction. Interlayer argon gas acts as a mechanical interferent to reaction, whilst interlayer nitrogen gas-modelled here with a novel interatomic potential-is shown to enhance heat production. These calculations also characterise compression wave dynamics in compression-loaded Ni/Al nanolaminates in greater detail than prior studies and illustrate significant qualitative and quantitative differences between extant embedded atom model (EAM) parameterisations of Ni/Al. Speeds of sound for each metal and EAM are also reported. These differences have implications for the interpretation and comparability of EAM-based modelling of Ni/Al reactions moving forward, and may also have wider implications for EAM modelling of intermetallic systems in general.

Hole-Pinned Defect Clusters for a Large Dielectric Constant up to GHz in Zinc and Niobium Codoped Rutile SnO2.

High permittivity materials for a gigahertz (GHz) communication technology have been actively sought for some time. Unfortunately, in most materials, the dielectric constant starts to drop as frequencies increase through the megahertz (MHz) range. In this work, we report a large dielectric constant of ∼800 observed in defect-mediated rutile SnO2 ceramics, which is nearly frequency and temperature independent over the frequency range of 1 mHz to 35 GHz and temperature range of 50-450 K. Experimental and theoretical investigations demonstrate that the origin of the high dielectric constant can be attributed to the formation of locally well-defined Zn2+-Nb4+ defect clusters, which create hole-pinned defect dipoles. We believe that this work provides a promising strategy to advance dipole polarization theory and opens up a direction for the design and development of high frequency, broadband dielectric materials for use in future communication technology.

The electronic structure of benzene from a tiling of the correlated 126-dimensional wavefunction.

The electronic structure of benzene is a battleground for competing viewpoints of electronic structure, with valence bond theory localising electrons within superimposed resonance structures, and molecular orbital theory describing delocalised electrons. But, the interpretation of electronic structure in terms of orbitals ignores that the wavefunction is anti-symmetric upon interchange of like-spins. Furthermore, molecular orbitals do not provide an intuitive description of electron correlation. Here we show that the 126-dimensional electronic wavefunction of benzene can be partitioned into tiles related by permutation of like-spins. Employing correlated wavefunctions, these tiles are projected onto the three dimensions of each electron to reveal the superposition of Kekulé structures. But, opposing spins favour the occupancy of alternate Kekulé structures. This result succinctly describes the principal effect of electron correlation in benzene and underlines that electrons will not be spatially paired when it is energetically advantageous to avoid one another.

Glycoluril derived cucurbituril analogues and the emergence of the most recent example: tiarauril.

Cucurbituril analogues can bear some of the chemical and physical characteristics of their parental origin and are derived wholly or in part from glycolurils (including homologues). The development of analogues is discussed from their earliest origins to the most recent developments, which includes deviations in binding properties and the inclusion of alternative molecular units in conjunction with glycolurils. Examples of alternative guest binding are discussed and compared to the behaviour of conventional cucurbituril.

Visualizing the 30-Dimensional Antisymmetrized Electronic Structure of Water: The Emergence of Lone Pairs.

The electronic structure of water is typically thought of as exhibiting lone pairs of electrons, described by some as "rabbit ears". This is not the universal view, and it does not mesh with an interpretation based on the one-electron wave functions that emerge from molecular orbital theory. Here, we show, by analyzing the 30-dimensional antisymmetrized wave function (Slater determinant) rather than the Hartree product, that the water wave function indeed exhibits equivalent lone pairs. The observed photoelectron spectrum is reconciled with this view, in terms of a relaxation of this structure upon the loss of an electron. Therefore, the lone-pair viewpoint is shown to be completely consistent with both the experimental results and the calculated wave function.

Electronic transitions of molecules: vibrating Lewis structures.

Since the conception of the electron pair bond, Lewis structures have been used to illustrate the electronic structure of a molecule in its ground state. But, for excited states, most descriptions rely on the concept of molecular orbitals. In this work we demonstrate a simple and intuitive description of electronic resonances in terms of localized electron vibrations. By partitioning the 3N-dimensional space of a many-electron wavefunction into hyper-regions related by permutation symmetry, chemical structures naturally result which correspond closely to Lewis structures, with identifiable single and double bonds, and lone pairs. Here we demonstrate how this picture of electronic structure develops upon the admixture of electronic wavefunctions, in the spirit of coherent electronic transitions. We show that π-π* transitions correspond to double-bonding electrons oscillating along the bond axis, and n-π* transitions reveal lone-pairs vibrating out of plane. In butadiene and hexatriene, the double-bond oscillations combine with in- and out-of-phase combinations, revealing the correspondence between electronic transitions and molecular normal mode vibrations. This analysis allows electronic excitations to be described by building upon ground state electronic structures, without the need for molecular orbitals.

Prominent out-of-plane diffraction in helium scattering from a methyl-terminated Si(111) surface.

Due to their electrochemical and oxidative stability, organic-terminated semiconductor surfaces are well suited to applications in, for example, photoelectrodes and electrochemical cells, which explains the lively interest in their detailed characterization. Helium atom scattering (HAS) is a useful tool to carry out such characterization. Here, we have simulated HAS in He/CH3-Si(111) based on density functional theory (DFT) potential energy surfaces (PESs) and multi-configuration time-dependent Hartree (MCTDH) dynamics. Our analysis of HAS shows that most diffraction taking place in this system corresponds to high-order out-of-plane peaks. This is a general trend that does not depend on the specific features of the simulations, such as the inclusion or not of the van der Waals long-range effects. This is the first and only He-surface system for which such huge out-of-plane diffraction has been described. This striking theoretical finding should encourage new experimental developments to confirm this previously unreported effect.

Non-adiabatic quantum molecular dynamics by the basis expansion leaping multi-configuration Gaussian (BEL MCG) method: Multi-set and single-set formalisms.

Non-adiabatic transitions are quite often of critical importance in chemical reactions. We have recently developed the basis expansion leaping multi-configuration Gaussian (BEL MCG) method to obtain time-propagated wave packets describing multidimensional reactive molecular systems such as quantum tunneling [T. Murakami and T. J. Frankcombe, J. Chem. Phys. 149, 134113 (2018)]. In this work, we develop BEL MCG for multiple electronic state problems. We present two formalisms for the BEL MCG description of multi-state wave packets, namely, "multi-set" and "single-set." We pay particular attention to investigate what is required to yield accurate dynamics. When there is low population on an electronic state, it is important in the "multi-set" case that the reexpression on that electronic state is applied rigorously. The sharing of basis functions in the single-set approach leads to needing a lower number of basis functions than in the multi-set approach, making it preferable for direct dynamics.

Accurate quantum molecular dynamics for multidimensional systems by the basis expansion leaping multi-configuration Gaussian (BEL MCG) method.

Quantum phenomena are quite often of critical importance in chemical reactions. Thus the development of quantum molecular dynamics approaches is required to study the role of quantum effects such as tunnelling in chemical processes. The basis expansion leaping multi-configuration Gaussian (BEL MCG) method has been developed to obtain time-propagated wave packets describing reactive molecular systems. Here we examine the applicability of BEL MCG to double well problems in several dimensions. We pay particular attention to what is required to yield highly accurate dynamics with respect to several key features of the BEL MCG propagation. The importance of using basis functions of a width appropriate to the nature of the potential energy surface in the region of configuration space where each basis function is located is highlighted, which has implications for virtually all quantum molecular dynamics methods utilising Gaussian basis functions.

Photoactivity and Stability Co-Enhancement: When Localized Plasmons Meet Oxygen Vacancies in MgO.

Durability is still one of the key obstacles for the further development of photocatalytic energy-conversion systems, especially low-dimensional ones. Encouragingly, recent studies show that nanoinsulators such as SiO2 and MgO exhibit substantially enhanced photocatalytic durability than the typical semiconductor p25 TiO2 . Extending this knowledge, MgO-Au plasmonic defect nanosystems are developed that combine the stable photoactivity from MgO surface defects with energy-focusing plasmonics from Au nanoparticles (NPs), where Au NPs are anchored onto monodispersed MgO nanotemplates. Theoretical calculations reveal that the midgap defect (MGD) states in MgO are generated by oxygen vacancies, which provide the main avenues for upward electron transitions under photoexcitation. These electrons drive stable proton photoreduction to H2 gas via water splitting. A synergistic interaction between Au's localized plasmons and MgO's oxygen vacancies is observed here, which enhances MgO's photoactivity and stability simultaneously. Such co-enhancement is attributed to the stable longitudinal-plasmon-free Au NPs, which provide robust hot electrons capable of overcoming the interband transition barrier (≈1.8 eV) to reach proton reduction potential for H2 generation. The demonstrated plasmonic defect nanosystems are expected to open a new avenue for developing highly endurable photoredox systems for the integration of multifunctionalities in energy conversion, environmental decontamination, and climate change mitigation.

Calculating curly arrows from ab initio wavefunctions.

Despite being at the heart of chemical thought, the curly arrow notation of reaction mechanisms has been treated with suspicion-the connection with rigorous molecular quantum mechanics being unclear. The connection requires a view of the wavefunction that goes beyond molecular orbitals and rests on the most fundamental property of electrons. The antisymmetry of electronic wavefunctions requires that an N-electron wavefunction repeat itself in 3N dimensions, thus exhibiting tiles. Inspection of wavefunction tiles permits insight into structure and mechanism. Here, we demonstrate that analysis of the wavefunction tile along a reaction coordinate reveals the electron movements depicted by the curly arrow notation for several reactions. The Diels-Alder reaction is revealed to involve the separation and counter propagation of electron spins. This unprecedented method of extracting the movements of electrons during a chemical reaction is a breakthrough in connecting traditional depictions of chemical mechanism with state-of-the-art quantum chemical calculations.

Colossal permittivity behavior and its origin in rutile (Mg1/3Ta2/3)xTi1-xO2.

This work investigates the synthesis, chemical composition, defect structures and associated dielectric properties of (Mg2+, Ta5+) co-doped rutile TiO2 polycrystalline ceramics with nominal compositions of (Mg2+1/3Ta5+2/3) x Ti1-x O2. Colossal permittivity (>7000) with a low dielectric loss (e.g. 0.002 at 1 kHz) across a broad frequency/temperature range can be achieved at x = 0.5% after careful optimization of process conditions. Both experimental and theoretical evidence indicates such a colossal permittivity and low dielectric loss intrinsically originate from the intragrain polarization that links to the electron-pinned [Formula: see text] defect clusters with a specific configuration, different from the defect cluster form previously reported in tri-/pent-valent ion co-doped rutile TiO2. This work extends the research on colossal permittivity and defect formation to bi-/penta-valent ion co-doped rutile TiO2 and elucidates a likely defect cluster model for this system. We therefore believe these results will benefit further development of colossal permittivity materials and advance the understanding of defect chemistry in solids.

Bimetallic Ions Codoped Nanocrystals: Doping Mechanism, Defect Formation, and Associated Structural Transition.

Ionic codoping offers a powerful approach for modifying material properties by extending the selection of potential dopant ions. However, it has been a major challenge to introduce certain ions that have hitherto proved difficult to use as dopants (called "difficult-dopants") into crystal structures at high concentrations, especially through wet chemical synthesis. Furthermore, the lack of a fundamental understanding of how codopants are incorporated into host materials, which types of defect structures they form in the equilibrium state, and what roles they play in material performance, has seriously hindered the rational design and development of promising codoped materials. Here we take In3+ (difficult-dopants) and Nb5+ (easy-dopants) codoped anatase TiO2 nanocrystals as an example and investigate the doping mechanism of these two different types of metal ions, the defect formation, and their associated impacts on high-pressure induced structural transition behaviors. It is experimentally demonstrated that the dual mechanisms of nucleation and diffusion doping are responsible for the synergic incorporation of these two dopants and theoretically evidenced that the defect structures created by the introduced In3+, Nb5+ codopants, their resultant Ti3+, and oxygen vacancies are locally composed of both defect clusters and equivalent defect pairs. These formed local defect structures then act as nucleation centers of baddeleyite- and α-PbO2-like metastable polymorphic phases and induce the abnormal trans-regime structural transition of codoped anatase TiO2 nanocrystals under high pressure. This work thus suggests an effective strategy to design and synthesize codoped nanocrystals with highly concentrated difficult-dopants. It also unveils the significance of local defect structures on material properties.

The Formation of Defect-Pairs for Highly Efficient Visible-Light Catalysts.

Highly efficient visible-light catalysts are achieved through forming defect-pairs in TiO2 nanocrystals. This study therefore proposes that fine-tuning the chemical scheme consisting of charge-compensated defect-pairs in balanced concentrations is a key missing step for realizing outstanding photocatalytic performance. This research benefits photocatalytic applications and also provides new insight into the significance of defect chemistry for functionalizing materials.

Challenges facing an understanding of the nature of low-energy excited states in photosynthesis.

While the majority of the photochemical states and pathways related to the biological capture of solar energy are now well understood and provide paradigms for artificial device design, additional low-energy states have been discovered in many systems with obscure origins and significance. However, as low-energy states are naively expected to be critical to function, these observations pose important challenges. A review of known properties of low energy states covering eight photochemical systems, and options for their interpretation, are presented. A concerted experimental and theoretical research strategy is suggested and outlined, this being aimed at providing a fully comprehensive understanding.

Chemical bonding motifs from a tiling of the many-electron wavefunction.

A method is presented to partition the 3N-dimensional space of a many-electron wavefunction into hyper-regions related by permutation symmetry. These hyper-regions represent unit cells, or "tiles" of the wavefunction from which the wavefunction may be regenerated in its entirety upon application of the set of permutations of like-spin electrons. The method, wherein a Voronoi diagram is constructed from the (even permutations of the) average position of a swarm of Monte Carlo walkers sampling |Ψ|(2), determines a self-consistent partitioning of the wavefunction. When one of the identical 3N-dimensional Voronoi sites is projected onto the coordinates of each electron, chemical motifs naturally appear, such as core electrons, lone-pairs, single-bonds and banana-bonds. The structures determined for N2, O2, F2, and other molecules correspond to the double-quartet theory of Linnett. When the procedure is applied to C2, we arrive at an interpretation of its bonding in terms of a near triple bond with singlet-coupled outer electrons.

Deprotonation of Water Ligands in V, Cr, Mn, Fe, and Co Complexes Reduces Oxidation-Driven Carboxylate Ligand Frequency Shifts.

In Mn complexes, it has been shown that oxidation-driven changes in carboxylate ligand vibrations are suppressed, if a water or hydroxo ligand is simultaneously deprotonated. Deprotonation with oxidation has also been shown to greatly reduce the dependence of Mn complex redox energies on the oxidation state of the metal. We have here investigated the effect of oxidation with deprotonation on the carboxylate ligand frequencies of V, Cr, Mn, Fe, and Co complexes. The effects of anionic ligand substitution (instead of deprotonation) and solvent dielectric were also investigated to determine the mechanism that drives carboxylate frequency shifts. It is shown that the effect of deprotonation was similar for all of the metals tested in this study. C-O bond lengths and O-C-O angle changes in the carboxylate ligand were also reduced by deprotonation. Furthermore, the effect of anionic ligand substitution was similar to deprotonation in the suppression of carboxylate frequency shifts. These shifts were also reduced by increases in the solvent dielectric, in the absence of charge conservation through deprotonation. Therefore, we conclude that carboxylate frequency shifts are largely driven by electrostatic effects.