In this work, we discuss use of machine learning techniques for rapid prediction of detonation properties including explosive energy, detonation velocity, and detonation pressure. Further, analysis is applied to individual molecules in order to explore the contribution of bonding motifs to these properties. Feature descriptors evaluated include Morgan fingerprints, E-state vectors, a custom "sum over bonds" descriptor, and coulomb matrices. Algorithms discussed include kernel ridge regression, least absolute shrinkage and selection operator ("LASSO") regression, Gaussian process regression, and the multi-layer perceptron (a neural network). Effects of regularization, kernel selection, network parameters, and dimensionality reduction are discussed. We determine that even when using a small training set, non-linear regression methods may create models within a useful error tolerance for screening of materials.

The determination of crystal structures plays an important role for model testing and validation, and understanding intra and intermolecular interactions that influence crystal packing. Here, we report the molecular structure of two recently synthesized energetic molecules, 3,3-bis-isoxazole-5,5'-bis-methylene dinitrate (C8H6N4O8, BIDN) and bis-isoxazole tetramethylene tetranitrate (C10H8N6O14, BITN) determined by single crystal x-ray diffraction and solid state density functional theory (DFT). BIDN is composed of two planar alkyl nitrate groups (r.m.s deviation = 0.0004 (1) angstrom) bonded to two planar azole rings (r.m.s deviation = 0.001 (1) angstrom, whereas BITN is composed of four planar alkyl nitrate groups (average r.m.s deviation = 0.002 (1) angstrom) bonded to two planar azole rings (average r.m.s deviation = 0.002 (1) angstrom). The theoretical calculations predict very well the planarity of both the alkyl nitrate groups and rings for both compounds. Furthermore, they predict well the bond lengths and angles of both molecules with mean deviation values of 0.018 angstrom (BIDN) and 0.017 angstrom (BITN) and 0.481 degrees (BIDN) and 0.747 degrees (BITN). Overall, the DFT determined torsion angles agree well with those determined experimentally for both BIDN (average deviation = 1.139 degrees) and BITN (average deviation = 0.604 degrees). The theoretical cell constant values are in excellent agreement with those determined experimentally for both molecules, with the BIDN a cell value and beta angle showing the largest deviation, 2.1% and 1.3%, respectively. Contacts between the atoms N and H dominate the intermolecular interactions of BIDN, whereas contacts involving the atoms O and H dominate the BITN intermolecular interactions. Electrostatic potential calculations at the B3LYP/6-31G* level reveal BIDN exhibits a lower sensitivity to impact compared to BITN. Published by Elsevier B.V.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

In recent years, large investments into the research of semiconducting two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs) have elucidated interesting device related physical phenomena such as valleytronics [1], 2D superconductivity [2], 2D excitonic effects [3] and vertical tunneling [4]. TMDs offer layer-dependent chemical tunability of electronic and optoelectronic properties governed by interlayer van der Waals (vdW) forces [5]. Because of their layered nature, these low-dimensional materials can be combined to form multifunctional heterostructure materials exhibiting entirely new physical systems offering new degrees of flexibility in designing electronics, optoelectronics and other novel devices [6], [7]. In the last couple of years, the focus in the 2D materials research have shifted from exploration of proof-of-concept devices using mechanically exfoliated materials to more advanced device processing using high-quality large-scale growth based on advanced scalable vdW-epitaxy techniques such as powder vapor deposition (PVD) and chemical vapor deposition (CVD).

A promising approach for high speed and high power electronics is to integrate two-dimensional (2D) materials with conventional electronic components such as bulk (3D) semiconductors and metals. In this study we explore a basic integration step of inserting a single monolayer MoS2 (1L-MoS2) inside a Au/p-GaN junction and elucidate how it impacts the structural and electrical properties of the junction. Epitaxial 1L-MoS2 in the form of triangle domains are grown by powder vaporization on a p-doped GaN substrate, and the Au capping layer is deposited by evaporation. Transmission electron microscopy (TEM) of the van der Waals interface indicates that 1L-MoS2 remained distinct and intact between the Au and GaN and that the Au is epitaxial to GaN only when the 1L-MoS2 is present. Quantitative TEM analyses of the van der Waals interfaces are performed and yielded the atomic plane spacings in the heterojunction. Electrical characterization of the all-epitaxial, vertical Au/1L-MoS2/p-GaN heterojunctions enables the derivations of Schottky barrier heights (SBH) and drawing of the band alignment diagram. Notably, 1L-MoS2 appears to be electronically semi-transparent, and thus can be considered as a modifier to the Au contact rather than an independent semiconductor component forming a pn-junction. The I–V analysis and our first principles calculation indicated Fermi level pinning and substantial band bending in GaN at the interface. Lastly, we illustrate how the depletion regions are formed in a bipolar junction with an ultrathin monolayer component using the calculated distribution of the charge density across the Au/1L-MoS2/GaN junction.

Organophosphorus nerve agents (NAs) irreversibly inhibit acetylcholinesterase (AChE), the enzyme responsible for breaking down the neurotransmitter acetylcholine (ACh). The over accumulation of ACh after NA exposure leads to cholinergic toxicity, seizure, and death. Current medical countermeasures effectively mitigate peripheral symptoms, however; the brain is often unprotected. Alternative acute treatment with the adenosine A1 receptor agonist N6-cyclopentyladensosine (CPA) has previously been demonstrated to prevent AChE inhibition as well as to suppress neuronal activity. The mechanism of AChE protection is unknown. To elucidate the feasibility of potential CPA-AChE interaction mechanisms, we applied a truncated molecular model approach and density functional theory. The candidate mechanisms studied are reversible enzyme inhibition, enzyme reactivation, and NA blocking prior to enzyme conjugation. Our thermodynamic data suggest that CPA can compete with the NAs sarin and soman for the active site of AChE, but may, in contrast to NAs, undergo back-reaction. We found a strong interaction between CPA and NA conjugated AChE, making enzyme reactivation unlikely but possibly allowing for CPA protection through the prevention of NA aging. The data also indicates that there is an affinity between CPA and unbound NAs. The results from this study support the hypothesis that CPA counters NA toxicity via multiple mechanisms and is a promising therapeutic strategy that warrants further development.

Optical charge state switching was previously observed in photoluminescence experiments for the divacancy defect in $4H$-SiC. The participating dark charge state could not be identified with certainty. We use constrained density-functional theory to investigate the mechanism of charge state conversion from the bright neutral charge state of the divacancy defect to the positive and negative charge states including corresponding recovery of the neutral charge state. While we can confirm that the positive charge state is dark, we do not find evidence that the negative charge state is dark. We compute similar absorption energies required for conversion of the neutral defect to both charge states. However, the formation of the positive charge state requires a series of excitations involving a 2-photon excitation, while the creation of the negative charge state is achieved through a single 2-photon process. Calculated absorption energies for the recovery of the neutral defect from the positive charge state fit the experimental value better than those from the negative charge state. Defect formation energies as a function of the Fermi energy show a very small Fermi energy range in which the negative charge state is most stable, while the positive charge state exhibits a wide stability range. Overall, our computational results give more support to the identification of the dark charge state as the positive over the negative charge state in the mechanism of optical charge state switching.

: In this report, an implementation of the Kubo-Greenwood formula for electrical conductivity, for use with the Quantum Espresso software package, is presented. Details of the implementation and instructions for execution are presented, and an example calculation of the frequency-dependent conductivity of liquid sodium is presented and compared to experimental results.

It has been previously demonstrated in a number of studies that the strength of boron carbide can be significantly reduced via introduction of a site vacancy. The site vacancies result in a discontinuous shock response at high pressure that is driven by bond formation between atoms which neighbor the vacancies. In this work, molecular dynamics simulations of boron suboxide (B6O) have been conducted to determine whether an analogous bond formation occurs at high pressure producing discontinuities in the B6O Hugoniot. Hugoniot data for ideal boron suboxide, a defect structure of boron suboxide, and a carbon doped structure are provided and the structural response of the oxygen atoms within the B6O crystal with increasing shock pressure is presented.

Integration of two-dimensional (2D) and conventional (3D) semiconductors can lead to the formation of vertical heterojunctions with valuable electronic and optoelectronic properties. Regardless of the growth stacking mechanism implemented so far, the quality of the formed heterojunctions is susceptible to defects and contaminations mainly due to the complication involved in the transfer process. We utilize an approach that aims to eliminate the transfer process and achieve epitaxial vertical heterojunctions with low defect interfaces necessary for efficient vertical transport. Monolayers of MoS2 of approximately 2 μm domains are grown epitaxially by powder vaporization on GaN substrates forming a vertical 2D/3D heterojunction. Cross-sectional transmission electron microscopy (XTEM) is employed to analyze the in-plane lattice constants and van der Waals (vdW) gap between the 2D and 3D semiconductor crystals. The extracted in-plane lattice mismatch between monolayer MoS2 and GaN is only 1.2% which corresponds well to the expected mismatch between bulk MoS2 and GaN. The vdW gap between MoS2 and GaN, extracted from the XTEM measurements, is consistent with the vdW gap of 3.1 Å predicted by our first principles calculations. The effect of monolayer (1L) MoS2 on the electrical characteristics of 2D/3D semiconductor heterojunctions was studied using conductive atomic force microscopy (CAFM). The electrical current across the CAFM-tip/1L-MoS2/GaN vertical junctions is dominated by the tip/GaN interface of both n- and p-doped GaN. This electronic transparency of 1L-MoS2 tells us that a 2D crystal component has to be above a certain thickness before it can serve as an independent semiconductor element in 2D/3D heterojunctions.

In the past decade, a number of approaches have been developed to fix the failure of (semi) local density-functional theory (DFT) in describing intermolecular interactions. The performance of several such approaches with respect to highly accurate benchmarks is compared here on a set of separation-dependent interaction energies for ten dimers. Since the benchmarks were unknown before the DFT-based results were collected, this comparison constitutes a blind test of these methods.

We describe the development of a density-dependent transferable coarse-grain model of crystalline hexahydro-1,3,5-trinitro-s-triazine (RDX) that can be used with the energy conserving dissipative particle dynamics method. The model is an extension of a recently reported one-site model of RDX that was developed by using a force-matching method. The density-dependent forces in that original model are provided through an interpolation scheme that poorly conserves energy. The development of the new model presented in this work first involved a multi-objective procedure to improve the structural and thermodynamic properties of the previous model, followed by the inclusion of the density dependency via a conservative form of the force field that conserves energy. The new model accurately predicts the density, structure, pressure-volume isotherm, bulk modulus, and elastic constants of the RDX crystal at ambient pressure and exhibits transferability to a liquid phase at melt conditions.

In this work, a convergence acceleration method applicable to extended system molecular dynamics techniques for shock simulations of materials is presented. The method uses velocity scaling to reduce the instantaneous value of the Rankine-Hugoniot conservation of energy constraint used in extended system molecular dynamics methods to more rapidly drive the system towards a converged Hugoniot state. When used in conjunction with the constant stress Hugoniostat method, the velocity scaled trajectories show faster convergence to the final Hugoniot state with little difference observed in the converged Hugoniot energy, pressure, volume and temperature. A derivation of the scale factor is presented and the performance of the technique is demonstrated using the boron carbide armour ceramic as a test material. It is shown that simulation of boron carbide Hugoniot states, from 5 to 20GPa, using both a classical Tersoff potential and an ab initio density functional, are more rapidly convergent when the velocity scaling algorithm is applied. The accelerated convergence afforded by the current algorithm enables more rapid determination of Hugoniot states thus reducing the computational demand of such studies when using expensive ab initio or classical potentials.

The shock Hugoniot of boron carbide, from 0 to 80 GPa, has been obtained using first principles quantum mechanics (density functional theory) and molecular dynamics simulation. The Hugoniot for six different structures which vary by structure or stoichiometry were computed and compared to experimental data. The effect of stoichiometry, and structural variation within a given stoichiometry, are shown to have marked effects on the shock properties with some compositions displaying bilinear behavior in the computed shock velocity‐particle velocity profiles while others show a continuous Hugoniot curve with no evidence of a phase transition over the pressure range considered in this work. Two structures, B12(CBC) and B11Cp(CCB), have predicted phase transition pressures lying within the 40–50 GPa range suggested experimentally. It is shown that the phase transition is driven by deformation of the 3‐atom chain within the boron carbide crystal structure which induces a discontinuous volume change at the critical shock pressure. The effect of defects, in the form of chain vacancies, on the shock response is presented and the ability of shear to significantly lower the phase transition pressure, in accord with experimental observation, is demonstrated.

Different models for aluminum oxynitride (AlON) were calculated using density functional theory and optimized using an evolutionary algorithm. Evolutionary algorithm and density functional theory (DFT) calculations starting from several models of AlON with different Al or O vacancy locations and different positions for the N atoms relative to the vacancy were carried out. The results show that the constant anion model [McCauley et al., J. Eur. Ceram. Soc. 29(2), 223 (2009)] with a random distribution of N atoms not adjacent to the Al vacancy has the lowest energy configuration. The lowest energy structure is in a reasonable agreement with experimental X-ray diffraction spectra. The optimized structure of a 55 atom unit cell was used to construct 220 and 440 atom models for simulation cells using DFT with a Gaussian basis set. Cubic elastic constant predictions were found to approach the experimentally determined AlON single crystal elastic constants as the model size increased from 55 to 440 atoms. The pressure dependence of the elastic constants found from simulated stress-strain relations were in overall agreement with experimental measurements of polycrystalline and single crystal AlON. Calculated IR intensity and Raman spectra are compared with available experimental data.

A nonempirical, physics-based modeling and simulation (M&S) capability for energetic materials can only be obtained within a multiscale M&S framework that has a quantum mechanical foundation. In this chapter, quantum mechanical methods and applications to investigate these challenging materials will be given, along with descriptions of how these can be used to enable a quantum-informed multiscale M&S capability for energetic materials. Current capability gaps will be presented, along with suggestions for research advances needed to address these deficiencies.