Van Der Waals Corrections Research Articles

Pressure, directional dependent mechanical anisotropies and phase transition studies of β-5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO) and 2,4,6-triamino-1,3,5-trinitrobenzene (TATB)

This work highlights the effect of pressure ranging from 0 to 9 GPa on structural, directional dependent mechanical properties and unravel the previously unknown phase transitions of two important high energy molecular solids namely monoclinic-β-Nitrotriazole (NTO) and 2,4,6-triamino-1,3,5-trinitrobenzene (TATB). The projected augmented plane wave method with generalized gradient approximation Perdew–Burke–Ernzerhof functional with the D2 van der Waals corrections method of Grimme is used to reproduce the experimental data within ∼1% error. The structural optimization results reveal that β-NTO undergoes a previously unknown structural phase transition at 9 GPa which is evident from the abrupt change of calculated lattice vectors, volume (V), lattice angle β at 9 GPa. The single crystal elastic properties analysis also supports these findings and NTO voilate the Born’s mechanical stability criteria at 9 GPa. Besides to it, all the calculated volumetric and directional dependent shear modulus (G), bulk modulus (B), compressibility results of β-NTO in (100), (010), (001) planes also suggest a possible phase transition around 9 GPa. The directional dependent polycrystalline compressibility anisotropy analysis of TATB with pressure in (100), (010), (001) planes unreveal the origin of experimentally reported new phase transition around 4 GPa. The calculated Pugh B/G ratio suggests that, both the materials found to be brittle in the studied pressure range except NTO at 9 GPa. The degree of mechanical anisotropy of β-NTO found to increase with increasing pressure from (100)–(010)–(001) planes, while the TATB anisotropy results were found to be relatively small and stable. The Young’s modulus (E), Poisson’s ratio (σ), P-wave modulus, universal elastic anisotropy (A ), Chung-Buessen anisotropy (A ), Vickers hardness coefficient (H ), sound velocities ((V ) average, (V ) longitudinal, (V ) transverse) and (θ D ) Debye temperature are also predicted. The calculated intermolecular interaction strength contribution to the total Hirsh Field Surface at different pressures confirms the initial decomposition mechanism of NTO, TATB and the results are good in agreement with previous observations. Thus our work has accentuated the reasons behind the impact and friction sensitivity differences of β-NTO, TATB through the two new phase transitions.

Lithium decorated Ѱ-graphene as a potential hydrogen storage material: Density functional theory investigations

In the present scenario, hydrogen has become a prominent alternative of fossil fuel that motivated us to develop more advanced nanomaterials to store efficient hydrogen for future use. This paper studies the hydrogen storage performance of Li-decorated Ѱ-graphene by conducting density functional theory simulations involving van der Waals corrections. Psi (Ѱ)-graphene is an advanced two-dimensional carbon allotrope that comprises pentagon, hexagon, and heptagon carbon rings. The simulation outcome reveals that the Li atom is bounded strongly to Ѱ-graphene with −2.63 eV binding energy through 0.89e charge transfer from Li-2s orbital to C-2p orbitals. Ѱ-graphene system decorated with the Li atom can bind seven hydrogen molecules with a suitable average binding energy of −0.31 eV/H2, high H2 gravimetric capacity of 15.15 wt%, and optimal average desorption temperature of 384 K. As per the Bader charge portioning, ∼0.09e charge is transferred from the Li-2s orbital to the H-1s orbital after hydrogen adsorption on Li + Ѱ-graphene. The hydrogen molecules are adsorbed on Li + Ѱ-graphene via a polarization mechanism. The high Li diffusion barrier of 0.78 eV and structural integrity of Li + Ѱ-graphene configuration at high temperature predicts that the system is a suitable candidate for high-capacity hydrogen storage applications.

Quantifying the Structure of Water and Hydrated Monovalent Ions by Density Functional Theory-Based Molecular Dynamics.

The accurate description of the structures of water and hydrated ions is important in electrochemical desalination, ion separation, and supercapacitors. In this work, we present an ab initio atomistic simulation-based study to explore the structure of water and hydrated monovalent ions (Li+, Na+, K+, Rb+, F-, and Cl-) at ambient conditions using generalized gradient approximation (GGA)-based methods with and without van der Waals correction (PBE, PBE + D3, and revPBE + D3) and recently developed strongly constrained and appropriately normed (SCAN) meta-GGA. We find that both revPBE + D3 and SCAN can well capture the structure of bulk water with +30 K artificial high temperature in contrast to overstructuring water using PBE and PBE + D3. However, being the same as PBE + D3, revPBE + D3 overestimates the structure of the hydration shell, especially for monovalent cations. Surprisingly, SCAN can well match the experimental results of hydrated monovalent ions. Detailed structure analyzes of entropy reveal that the hydration shell under the level of PBE + D3 and revPBE + D3 is more disordered and looser than SCAN. The successful prediction of the flexible SCAN functional could facilitate the exploration of complex ionic processes in the aqueous phase, the interactions of hydrated ions with surfaces, and solvation states in nanopores at an accurate, efficient, predictive, and ab initio level.

Testing the r2SCAN Density Functional for the Thermodynamic Stability of Solids with and without a van der Waals Correction.

A central aim of materials discovery is an accurate and numerically reliable description of thermodynamic properties, such as the enthalpies of formation and decomposition. The r2SCAN revision of the strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation (meta-GGA) balances numerical stability with high general accuracy. To assess the r2SCAN description of solid-state thermodynamics, we evaluate the formation and decomposition enthalpies, equilibrium volumes, and fundamental band gaps of more than 1000 solids using r2SCAN, SCAN, and PBE, as well as two dispersion-corrected variants, SCAN+rVV10 and r2SCAN+rVV10. We show that r2SCAN achieves accuracy comparable to SCAN and often improves upon SCAN's already excellent accuracy. Although SCAN+rVV10 is often observed to worsen the formation enthalpies of SCAN and makes no substantial correction to SCAN's cell volume predictions, r2SCAN+rVV10 predicts marginally less accurate formation enthalpies than r2SCAN, and slightly more accurate cell volumes than r2SCAN. The average absolute errors in predicted formation enthalpies are found to decrease by a factor of 1.5 to 2.5 from the GGA level to the meta-GGA level. Smaller decreases in error are observed for decomposition enthalpies. For formation enthalpies r2SCAN improves over SCAN for intermetallic systems. For a few classes of systems-transition metals, intermetallics, weakly bound solids, and enthalpies of decomposition into compounds-GGAs are comparable to meta-GGAs. In total, r2SCAN and r2SCAN+rVV10 can be recommended as stable, general-purpose meta-GGAs for materials discovery.

Coordination Engineering of Single‐Atom Iron Catalysts for Oxygen Evolution Reaction

AbstractDeveloping highly active oxygen evolution reaction (OER) catalyst is essential to improve the performance of a water electrolyzer, where single‐atom catalysts (SACs) have shown promising performance in recent years. However, while SACs with various coordination environment have become experimentally feasible, knowledge about the structure‐performance relationship of SACs is still very limited, especially for the emerging multiple p‐block element‐doped SACs substrates. Herein, by adjusting the coordination environment of single‐atom iron catalysts, 122 SACs with various substrates were designed for comprehensive analysis. Through spin‐polarized density functional theory calculations with van der Waals corrections, 52 stable single‐atom iron catalysts were identified. To analyze their OER performance, an OER volcano activity model was derived to predict their performance as the function of O* and HO* binding free energies. Interestingly, a Fe1B1C1N2‐pen structure was found to have an outstanding OER activity (with a theoretical overpotential of 0.40 V vs. reversible hydrogen electrode) due to its unique adsorption mode that strengthens HO‐bonding by forming the Fe−OH−B bond. Based on these results, the OER performance of 26 TM1/B1C1N2‐pen (TM=Ti−Zn, Zr−Cd, and Hf−Au) SACs were determined based on this unique coordination environment by B‐doping. Most importantly, this study shows that the OER performance of SACs can be significantly improved by tuning the coordination environment of an active metal center.

Understanding Trends in the NO Oxidation Activity of Single‐Atom Catalysts

Nitric oxide (NO) emitted by coal-fired power plants causes severe environmental pollution. Catalytic oxidation of NO to NO2 is a promising technology to reduce NO pollution. However, the NO oxidation capacities of conventional catalysts were usually insufficient at a low temperature due to the difficulty to activate molecular oxygen (O2). Herein, single-atom catalysts (SACs) with high O2 activation capacities are proposed for NO oxidation. To design optimal SACs, we explicitly analyzed eight 3d transition metal SACs using spin-polarized density functional theory calculations with van der Waals corrections (DFT+D3), transition state analysis, scaling relation analysis, reaction descriptor analysis, and microkinetic modeling. For the first time, we derived a volcano-shaped microkinetic model to unravel the activity trends in the NO oxidation of SACs as a function of oxygen adsorption energy. This model shows that a Fe1-N4-C structure possesses the best performance for NO oxidation among the 75 analyzed SACs, in excellent agreement with recent pioneering experimental studies. Based on this volcano model, we screened the rest of the SACs with a Metal1-N4-C structure and provided important design guidelines for NO oxidation catalysts.

Boosting Electrocatalytic Reduction of CO2 to HCOOH on Ni Single Atom Anchored WTe2 Monolayer.

Achieving efficient conversion of carbon dioxide (CO2 ) to formic acid (HCOOH) at mild conditions is a promising means to reduce greenhouse gas emission and mitigate the energy crisis. Herein, spin-polarized density functional theory calculations with van der Waals corrections (DFT+D3) are performed to analyze the catalytic activity of seven metals (Ti, Fe, Ni, Cu, Zn, In, and Sn) anchored on a tungsten ditelluride monolayer (M@WTe2 ) and screen favorable CO2 reduction pathways. These results demonstrate that Ni single atoms strongly bind to the WTe2 monolayer and exist in isolated form due to the high diffusion barriers. Also, Ni-anchored WTe2 monolayer (Ni@WTe2 ) possesses a considerably low limiting-potential (-0.11V vs reversible hydrogen electrode) to convert CO2 to HCOOH due to moderate OCHO adsorption energy and a suppressed competing hydrogen evolution reaction (HER). Therefore, Ni@WTe2 monolayer is a promising electrocatalytic material for the CO2 reduction reaction (CO2 RR). This study sheds light on strategies of designing single metal atom anchored WTe2 catalysts for improved CO2 RR performances.

High-pressure study of the aurophilic topological Dirac material AuI

We endeavour to explore the high-pressure study in the aurophilic AuI within the state-of-the-art of first principles. The impediment of expressing precise ground-state features of aurophilic compounds that had afflicted prior theoretical research has been resolved by incorporating van der Waals corrections (vdw). Mechanical and dynamical stability are ensured at ambient using the computed elastic constants and phonon dispersion curves. The dynamical instability is triggered by the application of pressure in AuI, as evidenced by the softening of an acoustic mode (Eu) at ∼7 GPa. Non-adherence of estimated elastic constants to the Born stability criterion at this pressure illustrates the system's mechanical instability. As previously demonstrated in experiments, the pressure-driven amorphization is rationalised by the phonon softening and elastic instability. Our calculations of the electronic band structure reveal an indirect bandgap (2.31 eV). Z2 invariants confirm that non-symmorphic AuI is a non-trivial Dirac material with the inclusion of spin-orbit coupling. Furthermore, a type-A hourglass dispersion with movable Dirac point is observed at the two-fold screw rotation axis (C2y). The pressure-dependent electronic structure reveal that the band topology is unaffected by pressure up to amorphous state. Our findings predict that this aurophilic class of material AuI possess exotic structural and electronic properties, encouraging further studies.

Theoretical understanding of water adsorption on stepped iron surfaces

Nanoscale zero-valent iron (nZVI) has attracted significant interest due to its high capability in groundwater remediation. Understanding the interaction between water and nZVI is essential to optimizing its performance. While stable stepped surfaces may play a unique role in nZVI during their practical applications due to their higher reactivity, the interactions between water and stepped surfaces are still rarely investigated. Here we applied density functional theory (DFT) to study the adsorption properties of water on stable stepped Fe(210) and Fe(211) surfaces with the consideration of the van der Waals correction. Our results revealed that the adsorption strength of molecular water is enhanced on surface Fe atoms with lower coordination numbers (CN). Accordingly, the water has a stronger interaction on the stepped surface after the dissociation into H and OH. The stepped surface can facilitate water dissociation by greatly reducing dissociation energy barriers. Since water adsorption and dissociation are the first stage of iron oxidation by water, our DFT results demonstrate that the water adsorption on stepped iron surface may determine the lifetime of nZVI. Our findings, therefore, suggests the passivation of stepped surface can be a practical approach to improve the performance of nZVI.

Effects of thermal, elastic, and surface properties on the stability of SiC polytypes

SiC polytypes have been studied for decades, both experimentally and with atomistic simulations, yet no consensus has been reached on the factors that determine their stability and growth. Proposed governing factors are temperature-dependent differences in the bulk energy, biaxial strain induced through point defects, and surface properties. In this work, we investigate the thermodynamic stability of the 3C, 2H, 4H, and 6H polytypes with density functional theory (DFT) calculations. The small differences of the bulk energies between the polytypes can lead to intricate changes in their energetic ordering depending on the computational method. Therefore, we employ and compare various DFT codes, i.e., vasp, cp2k, and fhi-aims; exchange-correlation functionals, i.e., LDA, PBE, PBEsol, PW91, HSE06, SCAN, and RTPSS; and nine different van der Waals (vdW) corrections. At $T=0$ K, 4H-SiC is marginally more stable than 3C-SiC, and the stability further increases with temperature by including entropic effects from lattice vibrations. Neither the most advanced vdW corrections nor strain on the lattice have a significant effect on the relative polytype stability. We further investigate the energies of the (0001) polytype surfaces that are commonly exposed during epitaxial growth. For Si-terminated surfaces, we find 3C-SiC to be significantly more stable than 4H-SiC. We conclude that the difference in surface energy is likely the driving force for 3C-nucleation, whereas the difference in the bulk thermodynamic stability slightly favors the 4H and 6H polytypes. In order to describe the polytype stability during crystal growth correctly, it is thus crucial to take into account both of these effects.

Electronic transport properties of compressed and stretched helicene-graphene nanostructures, a theoretical study

We present our ab initio simulations of strained helicene molecular junction. First, we present ab-initio models of [n]helicenes up to [100]helicene done by two different recent reactive force field molecular geometry optimization methods (ReaxFF CHOCsKNaClIFLi, 2019 and CHONSMgPNaCuCl, 2018). We used the [4]helicene and [10]helicene models for Density Functional Theory (DFT) electronic transport calculations of various graphene-helicene-graphene molecular junction configurations on Local Density Approximation (LDA) or Generalized Gradient Approach (GGA) level of exchange and correlation energy with Perdew-Zinger (PZ) or Perdew–Burke–Ernzerhof (PBE) functionals and Grimme DFT-D3 van der Waals correction. The electronic transport properties are studied by using of the Green's functions formalism. The electron transmissions and densities of states of [4]helicene (and [10]helicene) attached as a molecular junction in three different configurations to zigzag graphene nanoribbon (ZGNR) electrodes of three different widths were obtained. The structures are analyzed while relaxed, compressed or stretched. The current-voltage and current-strain characteristics are presented and results are compared and discussed. The work is supported by a short review summarizing some of the recent studies related to graphene, helicene, and molecular junctions under strain.

Study of the reversible hydrogen storage performance of Ti-decorated hexagonal B36 by DFT calculations with van der Waals corrections

In this work, we report on the study of the hydrogen storage capability of titanium (Ti) decorated B36 nanosheets using density functional theory (DFT) calculations with van der Waals corrections. Ti atoms are strongly bonded to the surface of B36 with a binding energy of 6.23 eV, which exceeds the bulk cohesive energy of crystalline Ti. Ti-decorated B36 (2Ti@B36) can reversibly adsorb up to 12 H2 molecules with a hydrogen storage capacity of 4.75 wt % and average adsorption energy between 0.361 and 0.674 eV/H2. The values of desorption temperature and the results of molecular dynamics simulations enable to conclude that 2Ti@B36 is a perspective reversible material for hydrogen storage under real conditions.

Better force fields start with better data: A data set of cation dipeptide interactions

We present a data set from a first-principles study of amino-methylated and acetylated (capped) dipeptides of the 20 proteinogenic amino acids – including alternative possible side chain protonation states and their interactions with selected divalent cations (Ca2+, Mg2+ and Ba2+). The data covers 21,909 stationary points on the respective potential-energy surfaces in a wide relative energy range of up to 4 eV (390 kJ/mol). Relevant properties of interest, like partial charges, were derived for the conformers. The motivation was to provide a solid data basis for force field parameterization and further applications like machine learning or benchmarking. In particular the process of creating all this data on the same first-principles footing, i.e. density-functional theory calculations employing the generalized gradient approximation with a van der Waals correction, makes this data suitable for first principles data-driven force field development. To make the data accessible across domain borders and to machines, we formalized the metadata in an ontology.

Boosting Electrocatalytic CO 2 Reduction with Conjugated Bimetallic Co/Zn Polyphthalocyanine Frameworks

Boosting Electrocatalytic CO ₂ Reduction with Conjugated Bimetallic Co/Zn Polyphthalocyanine Frameworks

Note: Reproducibility of potential energy surfaces of organic/metal interfaces on the example of PTCDA on Ag(111).

We recently published a benchmark study of common local, semi-local, and non-local exchange correlation functionals in combination with various van der Waals (vdW) corrections, where we investigated the reproducibility of the potential energy surface of perylenetetracarboxylic dianhydride on Ag(111). This Note presents an additional benchmark of the recently developed non-local many body dispersion (MBD-NL) vdW correction, coupled with the Perdew-Burke-Ernzerhof (PBE) functional. We find that this computation method shows similar performance as the established approaches. Notably, it yields very similar results as PBE + MBD.

Electronic and interfacial properties of graphene/HfS2 van der Waals heterostructure

Using density functional theory calculations and the addition of van der Waals correction, the graphene/HfS2 heterojunction is constructed, and its electronic properties are examined thoroughly. This interface is determined as n-type ohmic, and the impacts of different amounts of interlayer distance and strain on the contact are shown using Schottky barrier height and electron injection efficiency. Dipole moment and work function of the interface are also altered when subjected to change in these two categories. The effects of an applied electric field on transforming the ohmic contact to Schottky is also investigated. The conclusions given can assist in the design and modeling of HfS2 based devices in the future.

The small gas activities on different number of nitrogen atom doping to cobalt embedded graphene

The adsorption mechanisms of small gases on CoNx (x = 2,3,4) embedded graphene surfaces are investigated using DFT calculation. Focusing Bader charge analysis, charge density difference, Density of State for surface analysis and partial density of state, the gas activities (CO,CO2,NO,NO2,NH3 and O2) are studied in Quantum Esspresso software with Van der Waals correction. The adsorption energy, charge transfer, gas height from the surfaces, bond lengths, and magnetic momentum are calculated to understand the gas adsorption mechanisms. Doping nitrogen to graphene surface changes the gas adsorption system. Doping/removing nitrogen to/from the surface can increase the stability of gas on the modified graphene sheet. The study exhibits that the most stable gas is NO2 on CoNx graphene with chemical interaction. The weak interactions are observed on NH3 and CO2 molecules. These modified graphene sheets are promising materials as gas sensors.

Spin polarized edge states in TlGaS[formula omitted] nanoribbons

We have investigated TlGaS2 single-layer and TlGaS2 nanoribbons using density functional theory (DFT). Phonon dispersion curves calculated within the density functional perturbation theory indicate that the TlGaS2 single-layer is dynamically stable. Starting from optimized TlGaS2 single-layers, unterminated and terminated ribbon morphologies with varying edge Tl coverages were constructed. In order to account for the weak interaction between Tl and S atoms, van der Waals correction to DFT was employed. The electronic and magnetic structures of TlGaS2 ribbons are modulated by the edge morphology. Dangling bond, non-bonding, and free-electron-like edge states are formed. The ribbons have metallic conduction channels stemming from parabolic bands crossing the Fermi level. The edge states appearing in the fundamental energy gap induce spin polarization at the edges. The TlGaS2 nanoribbons provide two magnetic conduction channels along the Tl chains, and thus the nanoribbon acts as two magnetic nanowires.

Dependency of sliding friction for two-dimensional systems on electronegativity

We study the role of electronegativity in sliding friction for five different two-dimensional (2D) monolayer systems using ab initio calculations within density functional theory with van der Waals corrections. We show that the friction depends strongly on the involved atoms' electronegativity difference. All the studied systems exhibit almost the same magnitude of the friction force when sliding along the nonpolar path, independent of the material and the surface structures. In contrast, for sliding friction along a polar path, the friction force obeys a universal linear scaling law where the friction force is proportional to the electronegativity difference of its constituent atoms. We show that atomic dipoles in the 2D monolayers, induced by the electronegativity difference, enhance the corrugation of the charge distribution and increase the sliding barriers accordingly. Our studies reveal that electronegativity plays an important role in the friction of low-dimensional systems, and provides a strategy for designing nanoscale devices.

Two-dimensional ordering governs the overpotential of Li intercalation and plating on graphene and its variants

In this work, the Li ordering and its influence on Li intercalation and plating on graphite, bilayer, and single-layer graphene are investigated by first-principles calculation with two-dimensional cluster expansion and van der Waals corrections. The results show that Li intercalation has a multistage feature for graphite and bilayer graphene at Li concentrations from C2 to LiC6. Beyond LiC6, Li atoms are crowded in graphite and bilayer graphene, resulting in a negative discharge voltage. The calculated overpotential indicates Li plating easily happens on graphite but is unlikely on bilayer graphene. For single-layer graphene, Li atoms uniformly cover the graphene surface from C2 to LiC4 with the presence of voltage stages, while forming an atomic island at a higher Li concentration. Our findings not only give a good recount on recent Li plating phenomena in Li-ion batteries but also provide a rationale for circumventing those side reactions on graphene and its variants.