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

The structural and vibrational properties of two polymorphs of TiO, 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 TiO.

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 Hitchhiker's Guide to the Wave Function∥.

The electronic wave function of molecules is 3-dimensional and inseparable in the coordinates of the electrons. Whereas molecular orbitals are often invoked to visualize the electronic structure, they are nonunique, with the same 3-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.

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.

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

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 SnO 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.

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.

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.

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.

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.

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.

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 SiO and MgO exhibit substantially enhanced photocatalytic durability than the typical semiconductor p25 TiO . 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.

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.

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.

Colossal permittivity behavior and its origin in rutile (MgTa)TiO.

This work investigates the synthesis, chemical composition, defect structures and associated dielectric properties of (Mg, Ta) co-doped rutile TiO polycrystalline ceramics with nominal compositions of (MgTa) Ti O. Colossal permittivity (>7000) with a low dielectric loss (e.g.

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.

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

Highly efficient visible-light catalysts are achieved through forming defect-pairs in TiO 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.

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.

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.

Deprotonation of Water/Hydroxo Ligands in Clusters Mimicking the Water Oxidizing Complex of PSII and Its Effect on the Vibrational Frequencies of Ligated Carboxylate Groups.

The IR absorptions of several first-shell carboxylate ligands of the water oxidizing complex (WOC) have been experimentally shown to be unaffected by oxidation state changes in the WOC during its catalytic cycle. Several model clusters that mimic the Mn4O5Ca core of the WOC in the S1 state, with electronic configurations that correspond to both the so-called "high" and "low" oxidation paradigms, were investigated. Deprotonation at W2, W1, or O3 sites was found to strongly reduce carboxylate ligand frequency shifts on oxidation of the metal cluster.

H2 Adsorption in a Porous Crystal: Accurate First-Principles Quantum Simulation.

A general method is presented for constructing, from ab initio quantum chemistry calculations, the potential energy surface (PES) for H2 absorbed in a porous crystalline material. The method is illustrated for the metal-organic framework material MOF-5. Rigid body quantum diffusion Monte Carlo simulations are used in the construction of the PES and to evaluate the quantum ground state of H2 in MOF-5, the zero-point energy, and the enthalpy of adsorption at 0 K.

Explicit calculation of the excited electronic states of the photosystem II reaction centre.

The excited states of sets of the cofactors found in the photosystem II reaction centre have been calculated directly as a multi-monomer supermolecule for the first time. Time-dependent density functional theory was used with the CAM-B3LYP functional. Multiple excited states for each cofactor were found at lower energies than the lowest energy state corresponding to charge transfer states (in which an electron is shifted from one cofactor to another).

Using Hessian update formulae to construct modified Shepard interpolated potential energy surfaces: application to vibrating surface atoms.

Modified Shepard interpolation based on second order Taylor series expansions has proven to be a flexible tool for constructing potential energy surfaces in a range of situations. Extending this to gas-surface dynamics where surface atoms are allowed to move represents a substantial increase in the dimensionality of the problem, reflected in a dramatic increase in the computational cost of the required Hessian (matrix of second derivatives) evaluations. This work demonstrates that using approximate Hessians derived from well known Hessian update formulae and a single accurate Hessian can provide an effective way to avoid this expensive accurate Hessian determination.

Ab Initio modeling of the effect of oxidation coupled with HnO deprotonation on carboxylate ligands in Mn/Ca clusters.

Oxidation of some manganese complexes containing both carboxylate and water/hydroxo ligands does not result in changes to the carboxylate stretching frequencies. The water oxidizing complex of photosystem II is one motivating example. On the basis of electronic structure theory calculations, we here suggest that the deprotonation of water or hydroxo ligands minimizes changes in the vibrational frequencies of coligating carboxylates, rendering the carboxylate modes "invisible" in FTIR difference spectroscopy.