Multiemission of Ce3+ from a Single Crystallographic Site Induced by Disordering of Ions.

White-light emission with a single activator is an attractive function of phosphors. In this work, we investigated the photoluminescence properties of Ca5.7Y1.3Si7O16.7N3.3, which is a compound denoted as Ca4+xY3-xSi7O15+xN5-x discovered by our group, with Ce-activation using optical measurements and density functional theory (DFT) calculation. Samples showed a tunable emission from purple to white under ultraviolet (UV) light. In this compound, Ca and Y as well as anions are distributed disorderly, and Ca/Y ions occupy two crystallographically distinct sites; those sites are possible sites for Ce substitution. DFT calculation and structural refinement revealed that the tunable emission was generated by Ce at the crystallographically equivalent site but with distinct local structures caused by the disordering of cations and anions. As far as we know, this is the first report about a white-light-emitting phosphor with only Ce activation.

Locality error free effective core potentials for 3d transition metal elements developed for the diffusion Monte Carlo method.

Pseudopotential locality errors have hampered the applications of the diffusion Monte Carlo (DMC) method in materials containing transition metals, in particular oxides. We have developed locality error free effective core potentials, pseudo-Hamiltonians, for transition metals ranging from Cr to Zn. We have modified a procedure published by some of us in Bennett et al. [J. Chem. Theory Comput. 18, 828 (2022)]. We carefully optimized our pseudo-Hamiltonians and achieved transferability errors comparable to the best semilocal pseudopotentials used with DMC but without incurring in locality errors. Our pseudo-Hamiltonian set (named OPH23) bears the potential to significantly improve the accuracy of many-body-first-principles calculations in fundamental science research of complex materials involving transition metals.

Stiffer Bonding of Armchair Edge in Single-Layer Molybdenum Disulfide Nanoribbons.

The physical and chemical properties of nanoribbon edges are important for characterizing nanoribbons and applying them in electronic devices, sensors, and catalysts. The mechanical response of molybdenum disulfide nanoribbons, which is an important issue for their application in thin resonators, is expected to be affected by the edge structure, albeit this result is not yet being reported. In this work, the width-dependent Young's modulus is precisely measured in single-layer molybdenum disulfide nanoribbons with armchair edges using the developed nanomechanical measurement based on a transmission electron microscope. The Young's modulus remains constant at ≈166 GPa above 3 nm width, but is inversely proportional to the width below 3 nm, suggesting a higher bond stiffness for the armchair edges. Supporting the experimental results, the density functional theory calculations show that buckling causes electron transfer from the Mo atoms at the edges to the S atoms on both sides to increase the Coulomb attraction.

Biophysical Properties of the Fibril Structure of the Toxic Conformer of Amyloid-ß42: Characterization by Atomic Force Microscopy in Liquid and Molecular Docking.

Alzheimer's disease is associated with the aggregation of the misfolded neuronal peptide, amyloid-ß42 (Aß42). Evidence has suggested that several reasons are responsible for the toxicity caused by the aggregation of Aß42, including the conformational restriction of Aß42. In this study, one of the toxic conformers of Aß42, which contains a Glu-to-Pro substitution (E22P-Aß42), was explored using atomic force microscopy and molecular docking to study the aggregation dynamics. We proposed a systematic model of fibril formation to better understand the molecular basis of conformational transitions in the Aß42 species. Our results demonstrated the formation of amorphous aggregates in E22P-Aß42 that are stem-based, network-like structures, while the formation of mature fibrils occurred in the less toxic conformer of Aß42, E22-Aß42, that are sphere-like flexible structures. A comparison was made between the biophysical properties of E22P-Aß42 and E22-Aß42 that revealed that E22P-Aß42 had greater stiffness, dihedral angle, number of ß sheets involved, and elasticity, compared with E22-Aß42. These findings will have considerable implications toward our understanding of the structural basis of the toxicity caused by conformational diversity in Aß42 species.

Evolutionary Algorithm Directed Synthesis of Mixed Anion Compounds LaF2 X (X=Br, I) and LaFI2.

Mixed-anion compounds have attracted growing attentions, but their synthesis is challenging, making a rational search desirable. Here, we explored LaF3 -LaX3 (X=Cl, Br, I) system using ab initio structure searches based on evolutionary algorithms, and predicted LaF2 X and LaFX2 (X=Br, I), which are respectively isostructural with LaHBr2 and YH2 I, consisting of layered La-F blocks with single and double ordered honeycomb lattices, separated by van der Waals gaps. We successfully synthesized these compounds: LaF2 Br and LaFI2 crystallize in the predicted structure, while LaF2 I is similar to the predicted one but with different layer stacking. LaF2 I exhibits fluoride ion conductivity comparable to that of non-doped LaF3 , and has the potential to show better ionic conductivity upon appropriate doping, given the theoretically lower diffusion energy barrier and the presence of soft iodine anions. This study shows the structure prediction using evolutionary algorithms will accelerate the discovery of mixed-anion compounds in future, in particular those with an ordered anion arrangement.

Toward Chemical Accuracy Using the Jastrow Correlated Antisymmetrized Geminal Power Ansatz.

Herein, we report accurate atomization energy calculations for 55 molecules in the Gaussian-2 (G2) set using lattice regularized diffusion Monte Carlo (LRDMC). We compare the Jastrow-Slater determinant ansatz with a more flexible JsAGPs (Jastrow correlated antisymmetrized geminal power with singlet correlation) ansatz. AGPs is built from pairing functions, which explicitly include pairwise correlations among electrons, and hence, this ansatz is expected to be more efficient in recovering the correlation energy. The AGPs wave functions are first optimized at the variational Monte Carlo (VMC) level, which includes both the Jastrow factor and the nodal surface optimization. This is followed by the LRDMC projection of the ansatz. Remarkably, for many molecules, the LRDMC atomization energies obtained using the JsAGPs ansatz reach chemical accuracy (∼1 kcal/mol), and for most other molecules, the atomization energies are accurate within ∼5 kcal/mol. We obtained a mean absolute deviation of 1.6 kcal/mol with JsAGPs and 3.2 kcal/mol with JDFT (Jastrow factor + Slater determinant with DFT orbitals) ansatzes. This work shows the effectiveness of the flexible AGPs ansatz for atomization energy calculations and electronic structure simulations in general.

First-Principles-Based Insight into Electrochemical Reactivity in a Cobalt-Carbonate-Hydroxide Pseudocapacitor.

Cobalt carbonate hydroxide (CCH) is a pseudocapacitive material with remarkably high capacitance and cycle stability. Previously, it was reported that CCH pseudocapacitive materials are orthorhombic in nature. Recent structural characterization has revealed that they are hexagonal in nature; however, their H positions still remain unclear. In this work, we carried out first-principles simulations to identify the H positions. We then considered various fundamental deprotonation reactions inside the crystal and computationally evaluated the electromotive forces (EMF) of deprotonation (V dp). Compared with the experimental potential window of the reaction (<0.6 V (vs saturated calomel electrode (SCE)), the computed V dp (vs SCE) value (3.05 V) was beyond the potential window, indicating that deprotonation never occurred inside the crystal. This may be attributed to the strong hydrogen bonds (H-bonds) that formed in the crystal, leading to structural stabilization. We further investigated the crystal anisotropy in an actual capacitive material by considering the growth mechanism of the CCH crystal. By associating our X-ray diffraction (XRD) peak simulations with experimental structural analysis, we found that the H-bonds formed between CCH planes (approximately parallel to the ab-plane) can result in 1-D growth (stacked along the c-axis). This anisotropic growth controls the balance between the total "non-reactive" CCH phases (inside the material) and the "reactive" hydroxide (Co(OH)2) phases (surface layers); the former stabilizes the structure, whereas the latter contributes to the electrochemical reaction. The balanced phases in the actual material can realize high capacity and cycle stability. The results obtained highlight the possibility of regulating the ratio of the CCH phase versus the Co(OH)2 phase by controlling the reaction surface area.

Mechanistic insights and importance of hydrophobicity in cationic polymers for cancer therapy.

Development of molecules that can be effectively used for killing cancer cells remains a research topic of interest in drug discovery. However, various limitations of small molecules and nanotechnology-based drug-delivery systems hinder the development of chemotherapeutics. To resolve this issue, this study describes the potential application of polymeric molecules as anticancer drug candidates. We describe the design and synthesis of novel anticancer polymers containing hydrophobic groups. We established the fact that the cationic homopolymer (PAMPTMA) does not show any anticancer activity on its own; however, the insertion of hydrophobic moieties in copolymers (PAMPTMA-r-BuMA, PAMPTMA-r-HexMA, and PAMPTMA-r-OctMA) enhances their anticancer activity with a very low IC50 value (60 µg mL-1 for HepG2 cells). Mechanistic investigations were carried out using LDH leakage assay, cellular uptake, DOSY NMR and molecular dynamics to study the interaction between the polymer and the cell membrane as well as the role of hydrophobicity in enhancing this interaction. The results demonstrated that polymers are attracted by the anionic cancer cell membrane, which then leads to the insertion of hydrophobic groups inside the cell membrane, causing its disruption and ultimate lysis of the cell. This study demonstrates a novel and better approach for the rational design and discovery of new polymeric anticancer agents with improved efficacy.

Anionic ordering in Pb2Ti4O9F2 revisited by nuclear magnetic resonance and density functional theory.

A combination of 19F magic angle spinning (MAS) nuclear magnetic resonance (NMR) and density functional theory (DFT) were used to study the ordering of F atoms in Pb2Ti4O9F2. This analysis revealed that F atoms predominantly occupy two of the six available inequivalent sites in a ratio of 73 : 27. DFT-based calculations explained the preference of F occupation on these sites and quantitatively reproduced the experimental occupation ratio, independent of the choice of functional. We concluded that the Pb atom's 6s2 lone pair may play a role (∼0.1 eV per f.u.) in determining the majority and minority F occupation sites with partial density of states and crystal orbital Hamiltonian population analyses applied to the DFT wave functions.

Shry: Application of Canonical Augmentation to the Atomic Substitution Problem.

A common approach for studying a solid solution or disordered system within a periodic ab initio framework is to create a supercell in which certain amounts of target elements are substituted with other elements. The key to generating supercells is determining how to eliminate symmetry-equivalent structures from many substitution patterns. Although the total number of substitutions is on the order of trillions, only symmetry-inequivalent atomic substitution patterns need to be identified, and their number is far smaller than the total. Our developed Python software package, which is called Shry (Suite for High-throughput generation of models with atomic substitutions implemented by Python), allows the selection of only symmetry-inequivalent structures from the vast number of candidates based on the canonical augmentation algorithm. Shry is implemented in Python 3 and uses the CIF format as the standard for both reading and writing the reference and generated sets of substituted structures. Shry can be integrated into another Python program as a module or can be used as a stand-alone program. The implementation was verified through a comparison with other codes with the same functionality, based on the total numbers of symmetry-inequivalent structures, and also on the equivalencies of the output structures themselves. The provided crystal structure data used for the verification are expected to be useful for benchmarking other codes and also developing new algorithms in the future.

Making the most of data: Quantum Monte Carlo postanalysis revisited.

In quantum Monte Carlo (QMC) methods, energy estimators are calculated as (functions of) statistical averages of quantities sampled during a calculation. Associated statistical errors of these averages are often estimated. This error estimation is not straightforward and there are several choices of the error estimation methods. We evaluate the performance of three methods (the Straatsma method, an autoregressive model, and a blocking analysis based on von Neumann's ratio test for randomness) for the energy time series given by three QMC methods [diffusion Monte Carlo, full configuration interaction Quantum Monte Carlo (FCIQMC), and coupled cluster Monte Carlo (CCMC)]. From these analyses, we describe a hybrid analysis method which provides reliable error estimates for a series of various lengths of FCIQMC and CCMC's time series. Equally important is the estimation of the appropriate start point of the equilibrated phase. We establish that a simple mean squared error rule method as described by White [K. P. White, Jr., Simulation 69(6), 323 (1997)10.1177/003754979706900601] can provide reasonable estimations.

Computational Design to Suppress Thermal Runaway of Li-Ion Batteries via Atomic Substitutions to Cathode Materials.

The cathode material of a lithium-ion battery is a key component that affects durability, capacity, and safety. Compared to the LiCoO2 cathode material (the reference standard for these properties), LiNiO2 can extract more Li at the same voltage and has therefore attracted considerable attention as a material that can be used to obtain higher capacity. As a trade-off, it undergoes pyrolysis relatively easily, leading to ignition and explosion hazards, which is a challenge associated with the application of this compound. Pyrolysis has been identified as a structural phase transformation of the layered rocksalt structure → spinel → cubic rocksalt. Partial substitution of Ni with various elements can reportedly suppress the transformation and, hence, the pyrolysis. It remains unclear which elemental substitutions inhibit pyrolysis and by what mechanism, leading to costly material development that relies on empirical trial and error. In this study, we developed several possible reaction models based on existing reports, estimated the enthalpy change associated with the reaction by ab initio calculations, and identified promising elemental substitutions. The possible models were narrowed down by analyzing the correlations of the predicted dependence of the reaction enthalpies on elemental substitutions, compared between different reaction models. According to this model, substitution by P and Ta affords the highest enthalpy barrier between the initial (layered rocksalt) and the final (cubic rocksalt) structures but promotes the initial transformation to spinel as a degradation. Substitution by W instead generates the barrier to the final (preventing dangerous incidents) process, as well as for the initial degradation to spinel; therefore, it is a promising strategy to suppress the predicted pyrolysis.

Stepwise copolymerization of polybenzimidazole for a low dielectric constant and ultrahigh heat resistance.

Bio-based polymer materials having great potential due to the depletion of fossil-fuel resources have been applied as single-use and medicinal materials but their low thermomechanical resistance have limited wider applications. Here, ultrahigh thermoresistant bio-based terpolymers with a low dielectric constant, comprising polybenzimidazole and poly(benzoxazole-random-aramid), were prepared by a method involving stepwise polycondensation of three monomers, 3,4-diaminobenzoic acid for benzimidazoles, 3-amino-4-hydroxylbenzoic acid for benzoxazoles, and 4-aminobenzoic acid for aramids. For optimized monomer compositions, the obtained terpolymers exhibited dielectric constants lower than 3, and a 10% mass loss at approximately 760 °C which is a temperature higher than that for any other polymer material reported so far. The high thermal degradation temperatures of the prepared terpolymers were a result of the high interaction enthalpies of hydrogen bonding between imidazole rings in the polymer chains, which were obtained from density functional theory calculations using trimer models. Furthermore, the applicability of the prepared terpolymers as a wire-coating material for a simple motor insulation was demonstrated, indicating that it has significant potential to be used as a thermostable material with a low dielectric constant (k).

Diffusion Monte Carlo evaluation of disiloxane linearisation barrier.

The disiloxane molecule is a prime example of silicate compounds containing the Si-O-Si bridge. The molecule is of significant interest within the field of quantum chemistry, owing to the difficulty in theoretically predicting its properties. Herein, the linearisation barrier of disiloxane is investigated using a fixed-node diffusion Monte Carlo (FNDMC) approach, which is one of the most reliable ab initio methods in accounting for the electronic correlation. Calculations utilizing the density functional theory (DFT) and the coupled cluster method with single and double substitutions, including noniterative triples (CCSD(T)) are carried out alongside FNDMC for comparison. It is concluded that FNDMC successfully predicts the disiloxane linearisation barrier and does not depend on the completeness of the basis-set as much as DFT or CCSD(T), thus establishing its suitability.

Importance of Van der Waals Interactions in Hydrogen Adsorption on a Silicon-carbide Nanotube Revisited with vdW-DFT and Quantum Monte Carlo.

Density functional theory (DFT) is a valuable tool for calculating adsorption energies toward designing materials for hydrogen storage. However, dispersion forces being absent from the local/semi-local theory, it remains unclear as to how the consideration of van der Waals (vdW) interactions affects such calculations. For the first time, we applied diffusion Monte Carlo (DMC) to evaluate the adsorption characteristics of a hydrogen molecule on a (5,5) armchair silicon-carbide nanotube (H2-SiCNT). Within the DFT framework, we benchmarked various exchange-correlation functionals, including those recently developed for treating dispersion or vdW interactions. We found that the vdW-corrected DFT methods agree well with DMC, whereas the local (semilocal) functional significantly over (under)-binds. Furthermore, we fully optimized the H2-SiCNT geometry within the DFT framework and investigated the correlation between the structure and charge density. The vdW contribution to the adsorption was found to be non-negligible at ∼1 kcal/mol per hydrogen molecule, which amounts to 9-29% of the ideal adsorption energy required for hydrogen storage applications.

Peculiar Atomic Bond Nature in Platinum Monatomic Chains.

Metal atomic chains have been reported to change their electronic or magnetic properties by slight mechanical stimulus. However, the mechanical response has been veiled because of lack of information on the bond nature. Here, we clarify the bond nature in platinum (Pt) monatomic chains by our in situ transmission electron microscope method. The stiffness is measured with sub-N/m precision by quartz length-extension resonator. The bond stiffnesses at the middle of the chain and at the connection to the base are estimated to be 25 and 23 N/m, respectively, which are higher than the bulk counterpart. Interestingly, the bond length of 0.25 nm is found to be elastically stretched to 0.31 nm, corresponding to a 24% strain. Such peculiar bond nature could be explained by a novel concept of "string tension". This study is a milestone that will significantly change the way we think about atomic bonds in one-dimension.

A quantum annealing approach to ionic diffusion in solids.

We have developed a framework for using quantum annealing computation to evaluate a key quantity in ionic diffusion in solids, the correlation factor. Existing methods can only calculate the correlation factor analytically in the case of physically unrealistic models, making it difficult to relate microstructural information about diffusion path networks obtainable by current ab initio techniques to macroscopic quantities such as diffusion coefficients. We have mapped the problem into a quantum spin system described by the Ising Hamiltonian. By applying our framework in combination with ab initio technique, it is possible to understand how diffusion coefficients are controlled by temperatures, pressures, atomic substitutions, and other factors. We have calculated the correlation factor in a simple case with a known exact result by a variety of computational methods, including simulated quantum annealing on the spin models, the classical random walk, the matrix description, and quantum annealing on D-Wave with hybrid solver . This comparison shows that all the evaluations give consistent results with each other, but that many of the conventional approaches require infeasible computational costs. Quantum annealing is also currently infeasible because of the cost and scarcity of qubits, but we argue that when technological advances alter this situation, quantum annealing will easily outperform all existing methods.

Insights into the Mechanical and Electrical Properties of a Metal-Phosphorene Interface: An Ab Initio Study with a Wide Range of Metals.

Finding a metal contact with higher interface adhesion and lower contact resistivity is a major challenge in realizing 2D material-based field-effect transistors. The commonly used metals in the semiconductor industry have different interface chemistry with phosphorene. Although phosphorene FETs have been fabricated with gold, titanium, and palladium contacts, there are other metals with a better interface. In this work, using DFT, a systematic ab initio study of metal-phosphorene interfaces is carried out for a set of 18 potentially suitable metals with different resistivity, electronegativity, and work-function. The interface between these metals and phosphorene is studied to identify factors responsible for mechanical and electrical behavior of the metal contacts. The work of separation is calculated to measure the adhesion strength of the metal contacts, while the density of states, Schottky barrier height, tunnel barrier height, and the mid-interface charge density calculations are performed to analyze the electrical behavior. Both mechanical and electrical performance of the metal contacts are linked to the interface chemistry. Many important observations which deviate from the general trend are reported and explained.

Synthesis, Electronic Structure, and Physical Properties of Layered Oxypnictides Sr2ScCrAsO3 and Ba3Sc2Cr2As2O5.

New CrAs-based layered mixed-anion compounds Sr2ScCrAsO3 (SrScO-21113) and Ba3Sc2Cr2As2O5 (BaScO-32225) were synthesized, and their electronic structures and physical properties were investigated. The structures of these compounds comprise stacking of the anti-fluorite CrAs layer and perovskite-like SrScO or BaScO layers. The lattice constants of these compounds are relatively longer than those of the related compounds, such as BaCr2As2, owing to the insertion of a large perovskite blocking layer of SrScO/BaScO. While there are variations in the crystal structure of this system, such as 21113 and 32225, their chemical stability calculated by the first-principles calculations indicated that SrScO-21113 is energetically favorable compared to SrScO-32225. The formation energies of BaScO-32225 and BaScO-21113 are close to each other; in the experiment, while there was an indication of BaScO-21113 formation, only BaScO-32225 was formed as a single phase because of the low chemical stability of BaScO-21113. The partial density of states indicates that the majority of states are obtained from the 3d4-electrons of the Cr element hybridized modestly with p electrons at the Fermi energy. The magnetic properties of these compounds were paramagnetic, and they were different from related compounds, such as BaCr2As2, probably because of their long a-axis lengths. The temperature dependences of the electrical resistivities of both samples were in good agreement with the electronic band structure calculations. The variety of structures in the series of compounds with a CrAs layer results in different physical properties, and further development of new compounds may bring novel functionalities, such as superconductivity.

Ab Initio Evaluation of Complexation Energies for Cyclodextrin-Drug Inclusion Complexes.

We investigated the reliability of ab initio methods to predict the binding energies of molecular encapsulation complexes. Vast possibilities for the docking conformations were screened down to a couple of geometries using a semiempirical docking simulation. For the candidates, we applied density functional theory (DFT) with several exchange-correlation (XC) functionals to evaluate the binding energy. We carefully selected and compared the functionals to elucidate the role of the characteristic factors in achieving the XC effects. It is clarified that the improper combination in XC with D3 dispersion force correction leads to overbinding. For achieving a proper combination, the exchange interaction over the longer range to avoid the overbinding was found to be important.