High-level Density Research Articles

Machine learning-assisted DFT-prediction of pristine and endohedral doped (O and Se) Ge12C12 and Si12C12 nanostructures as anode materials for lithium-ion batteries

Nanostructured materials have gained significant attention as anode material in rechargeable lithium-ion batteries due to their large surface-to-volume ratio and efficient lithium-ion intercalation. Herein, we systematically investigated the electronic and electrochemical performance of pristine and endohedral doped (O and Se) Ge12C12 and Si12C12 nanocages as a prospective negative electrode for lithium-ion batteries using high-level density functional theory at the DFT/B3LYP-GD3(BJ)/6-311 + G(d, p)/GEN/LanL2DZ level of theory. Key findings from frontier molecular orbital (FMO) and density of states (DOS) revealed that endohedral doping of the studied nanocages with O and Se tremendously enhances their electrical conductivity. Furthermore, the pristine Si12C12 nanocage brilliantly exhibited the highest Vcell (1.49 V) and theoretical capacity (668.42 mAh g− 1) among the investigated nanocages and, hence, the most suitable negative electrode material for lithium-ion batteries. Moreover, we utilized four machine learning regression algorithms, namely, Linear, Lasso, Ridge, and ElasticNet regression, to predict the Vcell of the nanocages obtained from DFT simulation, achieving R2 scores close to 1 (R2 = 0.99) and lower RMSE values (RMSE < 0.05). Among the regression algorithms, Lasso regression demonstrated the best performance in predicting the Vcell of the nanocages, owing to its L1 regularization technique.

Engineering Rydberg-pair interactions in divalent atoms with hyperfine-split ionization thresholds

Quantum information processing with neutral atoms relies on Rydberg excitation for entanglement generation. While the use of heavy divalent or open-shell elements, such as strontium or ytterbium, has benefits due to their optically active core and a variety of possible qubit encodings, their Rydberg structure is generally complex. For some isotopes in particular, hyperfine interactions are relevant even for highly excited electronic states. We employ multichannel quantum defect theory to infer the Rydberg structure of isotopes with nonzero nuclear spin and perform nonperturbative Rydberg-pair interaction calculations. We find that due to the high level density and sensitivities to external fields, experimental parameters must be precisely controlled. Specifically in Sr87, we study an intrinsic Förster resonance, unique to divalent atoms with hyperfine-split thresholds, which simultaneously provides line stability with respect to external field fluctuations and enhanced long-range interactions. Additionally, we provide parameters for pair states that can be effectively described by single-channel Rydberg series. The explored pair states provide exciting opportunities for applications in the blockade regime, as well as for more exotic long-range interactions such as largely flat, distance-independent potentials. Published by the American Physical Society 2024

Comparative study of kilonova opacities for three elements of the sixth period (hafnium, osmium, and gold) from new atomic structure calculations in Hf I–IV, Os I–IV, and Au I–IV

Aims. It is now well established that a large amount of heavy (trans-iron) elements are produced during neutron star (NS) mergers. These elements can be detected in the spectra of the kilonova emitted from the post-merger ejected materials. Due to the high level densities that characterize the complex configurations belonging to heavy elements, thus giving rise to millions of absorption lines, the kilonova ejecta opacity is of significant importance. The elements that contribute the most to the latter are those with an unfilled nd subshell belonging to the fifth and the sixth rows of the periodic table, and those with an unfilled nf subshell belonging to the lanthanide and actinide groups. The aim of the present work is to make a new contribution to this field by performing large-scale atomic structure calculations in three specific sixth-row 5d elements, namely hafnium, osmium, and gold, in the first four charge stages (I–IV), and by computing the corresponding opacities, while focusing on the importance of the atomic models used. Methods. The pseudo-relativistic Hartree–Fock (HFR) method, including extended sets of interacting configurations, was used for the atomic structure and radiative parameter calculations, while the expansion formalism was used to estimate the opacities. Results. Theoretical energy levels, wavelengths, and oscillator strengths were computed for millions of spectral lines in Hf I–IV, Os I–IV, and Au I–IV ions, the reliability of these parameters being assessed through detailed comparisons with previously published experimental and theoretical results. The newly obtained atomic data were then used to calculate expansion opacities for typical kilonova conditions expected one day after the NS merger; these are a density of ρ = 10−13 g cm−3 and temperatures ranging from T = 5000 K to T = 15 000 K. Some agreements and differences were found when comparing our results with available data, highlighting the importance of using sufficiently complete atomic models for the determination of opacities.

Superalkalides based on stacked janus molecule with improved optical nonlinearity

Research is actively being carried out to design new materials with exceptional nonlinear optical (NLO) responses. In this context, a systematic investigation of designing novel hypothetical materials e.g., superalkalides based on stacked Janus dimer, all-cis-1,2,3,4,5,6-hexafluorocyclohexane (C6H6F6)2 has been reported using high-level density functional simulations. The superalkalides are designed here by doping alkali metals on the fluorine face and superalkalis on the hydrogen face of Janus dimer. The thermodynamic stability of newly designed complexes is demonstrated by the interaction energies (Eint), ranging between −13.64 and −33.12 kcal/mol. Moreover, the superalkalide nature of these complexes is proved by negative charge and HOMO on superalkalis in natural bond orbitals (NBO) and frontier molecular orbitals (FMO) analyses, respectively. Energy gaps (Eg) between HOMO and LUMO orbitals of the designed complexes lie in the range of 0.63–0.83 eV. UV–Vis analysis is performed to gain idea about the transparent region of electromagnetic spectrum for nonlinear activity. The nonlinear activity of the designed superalkalide(s) is studied by static first hyperpolarizability β0, dynamic first hyperpolarizability β(ω) and dynamic second hyperpolarizability γ(2- ω; ω, ω). The highest β0, β(ω), and γ(2- ω; ω, ω) ranges up to 1.1 × 107, 2.9 × 106 and 2.9 × 1012 au, respectively for designed complexes. A comprehensive examination of these superalkalides based on stacked Janus dimers with exceptionally high NLO response highlights their potential for use in nonlinear optical materials.

Superalkalis with Hydrogen as Central Electronegative Atom and their Possible Applications: Ab Initio and DFT Study.

Superalkalis are unusual species having ionization energies lower than that of the alkali metals. These species with various applications are of great importance in chemistry due to their low ionization energies and strong reducing property. A typical superalkali contains a central electronegative core decorated with excess metal ligands. In the quest for novel superalkalis, we have designed the superalkalis HLi2, HLiNa and HNa2 using hydrogen as central electronegative atom for the first time employing high level ab initio (CCSD(T), MP2) and density functional theory (ωB97X-D) methods. The superalkalis exhibit very low ionization energies, even lower than that of cesium. Stability of these species is verified from binding energy and dissociation energy values. The superalkalis are capable of reducing SO2, NO, CO2, CO and N2 molecules by forming stable ionic complexes and therefore can be used as catalysts for the reduction or activation of systems possessing very low electron affinities. The superalkalis form stable supersalts with tailored properties when interact with a superhalogen. They also show remarkably high non-linear optical responses, hence could have industrial applications. It is hoped that this work will enrich the superalkali family and spur further theoretical and experimental research in this direction.

Kato's theorem and ultralong-range Rydberg molecules

We consider nonadiabatic coupling in the “trilobite”-like long-range Rydberg molecules created by perturbing degenerate high-ℓ Rydberg states with a ground-state atom. Due to the flexibility granted by the high Rydberg level density, the avoided crossings between relevant potential energy curves can become extremely narrow, leading to highly singular nonadiabatic coupling. We find that the gap between the trilobite potential curve and neighboring “butterfly” or “dragonfly” potential curves can even vanish, as in a conical intersection, if the gap closes at an internuclear distance which matches a node of the s-wave radial wave function. This is an unanticipated outcome of Kato's theorem. Published by the American Physical Society 2024

Machine-Learned Potentials by Active Learning from Organic Crystal Structure Prediction Landscapes.

A primary challenge in organic molecular crystal structure prediction (CSP) is accurately ranking the energies of potential structures. While high-level solid-state density functional theory (DFT) methods allow for mostly reliable discrimination of the low-energy structures, their high computational cost is problematic because of the need to evaluate tens to hundreds of thousands of trial crystal structures to fully explore typical crystal energy landscapes. Consequently, lower-cost but less accurate empirical force fields are often used, sometimes as the first stage of a hierarchical scheme involving multiple stages of increasingly accurate energy calculations. Machine-learned interatomic potentials (MLIPs), trained to reproduce the results of ab initio methods with computational costs close to those of force fields, can improve the efficiency of the CSP by reducing or eliminating the need for costly DFT calculations. Here, we investigate active learning methods for training MLIPs with CSP datasets. The combination of active learning with the well-developed sampling methods from CSP yields potentials in a highly automated workflow that are relevant over a wide range of the crystal packing space. To demonstrate these potentials, we illustrate efficiently reranking large, diverse crystal structure landscapes to near-DFT accuracy from force field-based CSP, improving the reliability of the final energy ranking. Furthermore, we demonstrate how these potentials can be extended to more accurately model structures far from lattice energy minima through additional on-the-fly training within Monte Carlo simulations.

Photophysical properties of Pt(ii) complexes based on the benzoquinoline (bzq) ligand with OLED implication: a theoretical study.

In this study, we investigate photophysical properties of eight inorganic Pt(ii) complexes containing the bzq (benzoquinoline) ligand for OLED applications using high-level density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. We explore the radiative and non-radiative relaxation constants (k r, k nr), spin-orbit coupling (SOC) matrix elements, and spectral properties. To ensure compatibility between the host and guest compounds, we determine the HOMO and LUMO energy levels, as well as the triplet excitation energies of the selected systems, and evaluate their efficiency for OLED devices. Our findings indicate that all systems, except for 2a and 2b, exhibit a small S1-T1 energetic gap (ΔE ≤ 0.60 eV) and promising SOC matrix elements (25-93 cm-1), leading to a significant intersystem crossing (ISC) process. These complexes also show promising radiative relaxation rates (k r = ∼10-4 s-1) and high phosphorescent quantum yields (Φ > 30%). Thus, our results confirm that six out of the eight selected Pt(ii) complexes are promising candidates for use in the emitting layer (EML) of OLED devices as efficient green emitters.

Multimer Embedding Approach for Molecular Crystals up to Harmonic Vibrational Properties.

Accurate calculations of molecular crystals are crucial for drug design and crystal engineering. However, periodic high-level density functional calculations using hybrid functionals are often prohibitively expensive for the relevant systems. These expensive periodic calculations can be circumvented by the usage of embedding methods in which, for instance, the periodic calculation is only performed at a lower-cost level and then monomer energies and dimer interactions are replaced by those of the higher-level method. Herein, we extend such a multimer embedding approach to enable energy corrections for trimer interactions and the calculation of harmonic vibrational properties up to the dimer level. We evaluate this approach for the X23 benchmark set of molecular crystals by approximating a periodic hybrid density functional (PBE0+MBD) by embedding multimers into less expensive calculations using a generalized-gradient approximation functional (PBE+MBD). We show that trimer interactions are crucial for accurately approximating lattice energies within 1 kJ/mol and might also be needed for further improvement of lattice constants and hence cell volumes. Finally, the vibrational properties are already very well captured at the monomer and dimer level, making it possible to approximate vibrational free energies at room temperature within 1 kJ/mol.

The deterministic effect of laser energy density on the microstructures of novel duplex stainless steel fabricated via in-situ alloying in laser powder bed fusion

Distinctive microstructures and distributions of novel duplex (ferrite δ + austenite γ) stainless steel can be fabricated using in-situ alloying of Fe–Cr–Mn alloy and pure Ni powders by tuning laser energy density in powder bed fusion. At low-level energy density (∼60 J/mm3), ferrite solidification dominates and results in bimodal ferrite of columnar and equiaxed grains. At medium energy density (∼100 J/mm3), fine acicular Widmanstätten austenite satisfying the Nishiyama-Wassermann orientation relationship was produced within the coarse grains of δ, which is the predominant phase although ferrite and austenite solidification occur. At high-level energy density (∼194 J/mm3), ferrite and austenite solidification occur and the resultant microstructure consists of nearly equivalent fine equiaxed-grained δ and γ. The key difference in the microstructure was attributed to the different melt pool overlapping and types, which can be characterized by the size and local ratio of Cr equivalent (Creq) and Ni equivalent (Nieq) values (Creq/Nieq). The melt pool characteristics fundamentally determine the local solidification and transformation mode at different energy densities. The testing results have shown that the novel duplex stainless steel possesses excellent mechanical properties compared to conventionally-manufactured counterparts.

The Role of Water in the Adsorption of Nitro-Organic Pollutants on Activated Carbon.

The density functional theory (DFT) is applied to theoretically study the capture and storage of three different nitro polycyclic aromatic hydrocarbons, 4-nitrophenol, 2-nitrophenol, and 9-nitroanthracene by activated carbon, with and without the presence of water. These species are pollutants derived from vehicle and industry emissions. The modeling of adsorption is carried out at the molecular level using a high-level density functional theory with the B3LYP-GD(BJ)/6-31+G(d,p) level of theory. The adsorption energies of polluting gases considered isolated and in a humid environment are compared to better understand the role of water. The calculations reveal different possible pathways involving the formation of chemical bonds between adsorbent and adsorbate on the formation of intermolecular van der Waals interactions. The negative adsorption energy on AC for the three species is obtained when they are treated individually and in mixture with H2O. The basis-set superposition error, estimated using the counterpoise correction, varies the adsorption energies by 2-13%. Dispersion effects were also taken into account. The adsorption energy ranges from -10 to -414 kJ/mol suggesting a diversity of pathways. The resulting analysis suggests three preferred pathways for capture. The main pathway is physical interaction due to π-π stacking. Other means are capture due to the formation of hydrogen bonds resulting from water adsorbed on the surface and the simultaneous adsorption of pollutant and water where water can act as a link that promotes adsorption. The thermodynamic properties give a clue to the most eco-friendly approaches for molecular adsorption.

Construct exchange-correlation functional via machine learning.

Density functional theory has been widely used in quantum mechanical simulations, but the search for a universal exchange-correlation (XC) functional has been elusive. Over the last two decades, machine-learning techniques have been introduced to approximate the XC functional or potential, and recent advances in deep learning have renewed interest in this approach. In this article, we review early efforts to use machine learning to approximate the XC functional, with a focus on the challenge of transferring knowledge from small molecules to larger systems. Recently, the transferability problem has been addressed through the use of quasi-local density-based descriptors, which are rooted in the holographic electron density theorem. We also discuss recent developments using deep-learning techniques that target high-level abinitio molecular energy and electron density for training. These efforts can be unified under a general framework, which will also be discussed from this perspective. Additionally, we explore the use of auxiliary machine-learning models for van der Waals interactions.

Improved thermonuclear rate of42Ti(p,γ)43V and its astrophysical implication in therpprocess

Context.Accurate42Ti(p,γ)43V reaction rates are crucial for understanding the nucleosynthesis path of the rapid capture process (rpprocess) that occurs in X-ray bursts.Aims.We aim to improve the thermonuclear rates of42Ti(p,γ)43V based on more complete resonance information and a more accurate direct component, together with the recently released nuclear masses data. We also explore the impact of the newly obtained rates on therpprocess.Methods.We reevaluated the reaction rate of42Ti(p,γ)43V by the sum of the isolated resonance contribution instead of the Hauser-Feshbach statistical model. We used a Monte Carlo method to derive the associated uncertainties of new rates. The nucleosynthesis simulations were performed via the NuGrid post-processing code ppn.Results.The new rates differ from previous estimations due to the use of a series of updated resonance parameters and a direct S factor. Compared with the previous results from the Hauser-Feshbach statistical model, which assumes compound nucleus43V with a sufficiently high-level density in the energy region of astrophysical interest, large differences exist over the entire temperature region ofrp-process interest, up to two orders of magnitude. We consistently calculated the photodisintegration rate using our new nuclear masses via the detailed balance principle, and found the discrepancies among the different reverse rates are much larger than those for the forward rate, up to ten orders of magnitude at the temperature of 108K. Using a trajectory with a peak temperature of 1.95×109K, we performed therp-process nucleosynthesis simulations to investigate the impact of the new rates. Our calculations show that the adoption of the new forward and reverse rates result in abundance variations for Sc and Ca of 128% and 49%, respectively, compared to the variations for the statistical model rates. On the other hand, the overall abundance pattern is not significantly affected. The results of using new rates also confirm that therp-process path does not bypass the isotope43V.Conclusions.Our study found that the Hauser-Feshbach statistical model is inappropriate to the reaction rate evaluation for42Ti(p,γ)43V. The adoption of the new rates confirms that the reaction path of42Ti(p,γ)43V(p,γ)44Cr(β+)44V is a key branch of therpprocess in X-ray bursts.

Morphological and Coordination Modulations in Iridium Electrocatalyst for Robust and Stable Acidic OER Catalysis.

Proton exchange membrane water splitting (PEMWS) technology has high-level current density, high operating pressure, small electrolyzer-size, integrity, flexibility, and has good adaptability to the volatility of wind power and photovoltaics, but the development of both active and high stability of the anode electrocatalyst in acidic environment is still a huge challenge, which seriously hinders the promotion and application of PEMWS. In recent years, researchers have made tremendous attempts in the development of high-quality active anode electrocatalyst, and we summarize some of the research progress made by our group in the design and synthesis of PEMWS anode electrocatalysts with different nanostructures, and makes full use of electrocatalytic activity points to increase the inherent activity of Iridium (Ir) sites, and provides optimization strategies for the long-term non-decay of catalysts under high anode potential in acidic environments. At this stage, these research advances are expected to facilitate the research and technological progress of PEMWS, and providing some research ideas and references for future research on efficient and inexpensive PEMWS anode electrocatalysts.

High level abinitio and density functional study of TeF6+ and TeCl6+: Attainability of +7 oxidation state for tellurium.

Discovery of a new oxidation state for an element expands its chemistry. A high oxidation state, such as +7, is rare for sp-block elements except for halogens. In this study, we determined that Te can attain a +7 oxidation state through the existence of a distorted octahedron (DOH) structure of TeCl6+ based on coupled cluster singles and doubles with perturbative triples calculations. We propose a new type of isomerization that resembles pseudorotation. The octahedron structure of TeF6+ bearing one elongated axial bond isomerizes to a DOH via an associated pseudorotation.

Transformation of ICEBall to fIREBall for conversion electron spectroscopy

Conversion electron spectroscopy is a valuable tool in nuclear structure studies, but the complex spectra resulting from the high density of levels in the spectra of heavy nuclei make it difficult to extract the most beneficial information from singles measurements. This work reports on the transformation of ICEBall, which was a mini-orange spectrometer that was used in coincidence with various Germanium detector arrays, to the improved fInternal conveRsion Electron Ball (fIREBall) spectrometer for the improved measurements of E0 transitions and conversion electron coefficients. A combination of FreeCAD, COMSOL, and Geant4 simulations were implemented to optimize the geometries for the magnet filters used in the spectrometer to significantly improve the efficiency of electron collection in the energy range of interest 200 keV-1 MeV. The array of six mini-orange Si(Li) detectors are to be replaced with six new, thicker Si(Li) detectors. Experimental tests reported here show a peak improvement in absolute efficiency from 2.8% to 5.3% for the 482 keV K electron line in 207Bi for a single magnet filter and detector pair.

Computational Studies of Rubber Ozonation Explain the Effectiveness of 6PPD as an Antidegradant and the Mechanism of Its Quinone Formation.

The discovery that the commercial rubber antidegradant 6PPD reacts with ozone (O3) to produce a highly toxic quinone (6PPDQ) spurred a significant research effort into nontoxic alternatives. This work has been hampered by lack of a detailed understanding of the mechanism of protection that 6PPD affords rubber compounds against ozone. Herein, we report high-level density functional theory studies into early steps of rubber and PPD (p-phenylenediamine) ozonation, identifying key steps that contribute to the antiozonant activity of PPDs. In this, we establish that our density functional theory approach can achieve chemical accuracy for many ozonation reactions, which are notoriously difficult to model. Using adiabatic energy decomposition analysis, we examine and dispel the notion that one-electron charge transfer initiates ozonation in these systems, as is sometimes argued. Instead, we find direct interaction between O3 and the PPD aromatic ring is kinetically accessible and that this motif is more significant than interactions with PPD nitrogens. The former pathway results in a hydroxylated PPD intermediate, which reacts further with O3 to afford 6PPD hydroquinone and, ultimately, 6PPDQ. This mechanism directly links the toxicity of 6PPDQ to the antiozonant function of 6PPD. These results have significant implications for development of alternative antiozonants, which are discussed.

Analysis of defects and non-metallic inclusions distribution in high-strength TWIP steel Fe-25Mn-12Al-1.5C after electroslag remelting

TWIP steels belong to the list of the most innovative materials of our time due to the combination of a high mechanical characteristics level and low density. The most high-strength alloys usually contain about 25-30 wt. % manganese and about 10 wt. % aluminum. Production of such steels is complicated by the peculiarities of their chemical composition. Due to the high content of manganese and aluminum, they are prone to components liquation by density, have a greater number of shrinkage defects and an increased number of sulfides, nitrides and oxides non-metallic impurities. This determines the use of effective refining methods, which include electroslag remelting (ESR). The paper shows a comparison of Fe-25Mn-12Al-1.5C alloy structure, type and amount of non-metallic inclusions after induction melting and after refining electroslag remelting. Electron microscopy of the samples and local chemical analysis of the phases showed a large number of non-metallic inclusions — sulfides, phosphides, and oxynitrides. After refining process, it was shown that electroslag remelting contributes to a noticeable decrease of nitrogen and sulfur content, and as the result — it lowers the number of related of non-metallic inclusions. However, it seems to be an insufficiently effective method of refining materials like TWIP-steels. Relatively large size of the non-metallic inclusions, low phosphides refining ability and the crystallization conditions under which a directionally crystallized structure forms, may be noticed among the disadvantages of the ESR method. It was also established that in crystallizer zones, close to the bottom and walls, metal refines worse than its central volumes. Upper part of the ingot has shrinkage and sub-shrinkage zones enriched with gas-shrinkage defects, so it can be called a problem zone. In general, it is shown that the ESR method is not capable to solve a problem of refining high-manganese TWIP steels with a high aluminum content to the required extent.

Property and reactivity of polyselenides and polysulfides: a quantum chemistry study

High-level ab initio and density functional theory were applied to investigate property and reactivity of polyselenides and polysulfides. With CH3XnH (X = S or Se) we found that the deprotonation energy (and pKa ) of CH3SenH, ionization potential (IP) of , and CH3Sen−H bond dissociation energy (BDE) are lower than those of their sulfur counterparts. For n = 2, IPs and X−H BDEs are exceptionally small. The relative nucleophilicity of vary with substrates. However, CH3SenH are more nucleophilic than CH3SnH due to their due to their lower deprotonation energy. The nucleophilicity of selenolates versus thiolates are related to the chemical nature. Selenium is more electrophilic than sulfur.

Perfluoropropionic Acid-Driven Nucleation of Atmospheric Molecules under Ambient Conditions

A molecular-level understanding of the compositions and formation mechanism of secondary organic aerosols is important in the context of growing evidence regarding the adverse impacts of aerosols on the atmosphere and human health. The ever-growing emissions of pollutants and particulate matter in the atmosphere are a global concern. A particular class of pollutants, which are being important in this sense, are persistent organic pollutants (POPs) since they represent synthetic organic compounds with a long lifetime in the environment. Among the POPs, the perfluorinated compounds, such as perfluoroalkyl carboxylic acids (CnF2n+1COOH) or PFCAs, draw a lot of attention due to their adverse effect on human health. In the present work, we employ high-level density functional theory to investigate the electrostatic interaction of perfluoropropionic acid (C2F5COOH) or PFPA, a PFCA with n = 2, with well-known atmospheric molecules, namely, HCHO, HCOOH, CH3OH, H2SO4, and CH3SO3H [methanesulfonic acid (MSA)]. A detailed and systematic quantum chemical calculation has been performed to analyze the structural, energetic, electrical, and spectroscopic properties of several binary clusters in the context of atmospheric nucleation process. Our analysis shows that PFPA forms very stable hydrogen-bonded binary clusters with molecules like H2SO4 and MSA, which widely recognized atmospheric nucleation precursors. Scattering intensities of radiation (Rayleigh activities) are found to increase many fold when PFPA forms clusters. Analyses of the cluster-binding electronic energies and the free-energy changes associated with their formation at different temperatures indicate that PFPA could participate in the initial nucleation processes and contribute effectively to the new particle formation in the atmosphere.