Plane-wave Density Functional Theory Research Articles

First principles validation of energy barriers in Ni75Al25

Precipitates in nickel-based superalloys form during heat treatment on a time scale inaccessible to direct molecular dynamics simulation, but can be studied using kinetic Monte Carlo (KMC) modelling. This requires reliable values for the barrier energies separating distinct configurations over the trajectory of the system. In this study, we validate vacancy migration barriers found with the Activation-Relaxation Technique nouveau (ARTn) method in partially ordered Ni75Al25 with a monovacancy using published potentials for the atomic interactions against first-principles methods. In a first step, we confirm that the ARTn barrier energies agree with those determined with the nudged elastic band (NEB) method. As the number of atoms used in those calculations is too great for direct ab initio calculations, we cut the cell size to 255 atoms, thus controlling finite size effects. We then use the plane-wave density functional theory code CASTEP and its inbuilt NEB method in the smaller cells. This provides us with a continuous validation chain from first principles to KMC simulations with interatomic potentials (IPs). We evaluate the barrier energies of five further IPs with NEB, demonstrating that none yields values with sufficient reliability for KMC simulations, with some of them failing completely. This is a first step towards quantifying the errors incurred in KMC simulations of precipitate formation and evolution.

Enhanced figure of merit in two-dimensional ZrNiSn nanosheets for thermoelectric applications

A novel two-dimensional (2D) half-Heusler ZrNiSn nanosheet for thermoelectric applications was designed from bulk half-Heusler ZrNiSn through first-principles calculation. Investigation of bulk half-Heusler and 2D nanosheet ZrNiSn was performed with the Quantum Espresso code based on a density functional theory plane wave basis set. Electronic band structure and density of states calculations were used to study the confinement effects. On moving from bulk to 2D a change of structure is observed from face-centered cubic to trigonal due to confinement effects. The semiconducting nature of bulk ZrNiSn is undisturbed while moving to a 2D nanosheet; however, the band gap is widened from 0.46 to 1.3 eV due to the restricted motion of electrons in one direction. Compared with bulk ZrNiSn, 2D nanosheets were found to have a higher Seebeck coefficient a lower thermal conductivity and higher figure of merit, which makes 2D ZrNiSn nanosheets suitable for thermoelectric applications. Atomically thin 2D structures with a flat surface have the potential to form van der Waals heterojunctions, paving the way for device fabrication at the nanoscale level.

Strain-dependent one-dimensional confinement channels in twisted bilayer 1T′−WTe2

The low-symmetry and anistropic lattice of 1T′WTe2 is responsible for the existence of parallel one-dimensional channels in the moiré patterns of twisted bilayers. This gives the opportunity to explore moiré physics of a different nature to that widely observed in twisted bilayers of materials with hexagonal symmetries. Here, we combine plane-wave and linear-scaling density functional theory calculations to describe the electronic properties of twisted bilayer 1T′WTe2. We investigate the strain dependence of both aligned bilayer configurations across a range of stacking geometries, and of the twisted bilayer itself. For a small change in the lattice parameters of the constituent 1T′WTe2 monolayers, we find a very substantial increase in the moiré-induced striped electrostatic potential landscape in the twisted bilayer. We measure a peak-to-trough magnitude >200 meV between parallel linear channels, sufficient to induce Luttinger liquid behavior. Published by the American Physical Society 2024

First principles calculation and corrosion resistance study of Fe60CrxNi40-x medium entropy alloy

In this paper, Fe60CrxNi40-x medium entropy alloys (MEAs) were synthesized through vacuum arc melting. Based on the plane wave pseudo-potential and density functional theory, the solid solution structure is modeled by virtual crystal approximation (VCA). The first principle method was employed to examine how Cr content affects the phase structure, elastic constant, and elastic properties of the Fe60CrxNi40-x. Physical phase analysis of the MEAs by X-ray diffraction reveals that the Fe60CrxNi40-x maintain a single FCC solid solution structure. Meanwhile, the impact of Cr content on MEAs' resistance to corrosion in a Cl- setting was examined and contrasted with 316 L stainless steel. The result indicated that as the Cr content rises, the MEAs exhibit an extended passivation period, a reduced dimensional passivation current, and a more compact and stable passivation film. When considering the aggregate outcomes of pitting potential (Epit), self-corrosion potential (Ecorr), and corrosion current density (icorr) for 316LSS and MEAs, it's evident that Fe60CrxNi40-x in 3.5 wt% NaCl solution exhibits superior corrosion resistance compared to 316 L stainless steel, with Fe3 alloy leading in corrosion resistance among medium entropy alloys.

Enhanced hydrogen adsorption-desorption reversibility found on NiAl alloy: A first-principles study

Improving the hydrogen storage performance of carbon-based materials via transition metals incorporation has been proved experimentally, where control over the hydrogen adsorption kinetics and hydrogen surface diffusion is the key. In this work, we proved theoretically via density functional theory (DFT) how such incorporation promotes better hydrogen storage performance in Ni-based materials supported on graphene, in which static and dynamic behavior of the system were modeled by plane-wave DFT and ab initio molecular dynamics (AIMD), respectively. In addition, the hydrogen storage materials Ni10Al3 and Ni13 supported on graphene, denoted as Ni10Al3/GP and Ni13/GP, are modeled to realize the experimental observation on Ni3Al alloy under high hydrogen pressure. It was found that the Ni10Al3/GP alloy system exhibited better properties in twofold: (1) increased hydrogen adsorption capacity through providing an additional site for H2 molecular adsorption, particularly on the Al atoms, and (2) enhanced hydrogen desorption at ambient conditions observed from reversible H2 adsorption at all coverages with barrierless migration energy. Therefore, the proof from the DFT point-of-view provides valuable insight into exploring other metal alloy systems that can provide additional sites for molecular hydrogen adsorption and improve hydrogen adsorbate mobility.

Effect of Metal Carbides on Hydrogen Embrittlement: A Density Functional Theory Study

This study uses plane wave density functional theory (DFT) to investigate the effect of certain metal carbides (Niobium carbide, Vanadium carbide, Titanium carbide, and Manganese sulfide) on hydrogen embrittlement in pipeline steels. Our results predict that the interaction of hydrogen molecules with these metal carbides occurs in the long range with binding energy varying in the energy window [0.043 eV to 0.70 eV].In addition, our study shows the desorption of H2 molecules from these metal carbides in the chemisorptions. Since atomic state hydrogen interacts with NbC, VC, TiC, and MnS to cause embrittlement, we classified the strength of the hydrogen trapping as TiC + H > VC + H > NbC + H> MnS + H. In addition, our study reveals that the carbon site is a more favorable hydrogen-trapping site than the metal one.

Physical origin of enhanced electrical conduction in aluminum-graphene composites

The electronic and transport properties of aluminum-graphene composite materials were investigated using the ab initio plane wave density functional theory. The interfacial structure is reported for several configurations. In some cases, the face-centered aluminum (111) surface relaxes in a nearly ideal registry with graphene, resulting in a remarkably continuous interface structure. The Kubo–Greenwood formula and space-projected conductivity were employed to study electronic conduction in aluminum single- and double-layer graphene-aluminum composite models. The electronic density of states at the Fermi level is enhanced by the graphene for certain aluminum–graphene interfaces, thus improving electronic conductivity. In double-layer graphene composites, conductivity varies non-monotonically with temperature, showing an increase between 300 and 400 K at short aluminum-graphene distances, unlike the consistent decrease in single-layer composites.

Massively parallel implementation of iterative eigensolvers in large-scale plane-wave density functional theory

The Kohn-sham density functional theory (DFT) is a powerful method to describe the electronic structures of molecules and solids in condensed matter physics, computational chemistry and materials science. However, large and accurate DFT calculations within plane waves process a cubic-scaling computational complexity, which is usually limited by expensive computation and communication costs. The rapid development of high performance computing (HPC) on leadership supercomputers brings new opportunities for developing plane-wave DFT calculations for large-scale systems. Here, we implement parallel iterative eigensolvers in large-scale plane-wave DFT calculations, including Davidson, locally optimal block preconditioned conjugate gradient (LOBPCG), projected preconditioned conjugate gradient (PPCG) and the Chebyshev subspace iteration (CheFSI) algorithms, and analyze the performance of these algorithms in massively parallel plane-wave computing tasks. We adopt a two-level parallelization strategy that combines the message passing interface (MPI) with open multi-processing (OpenMP) parallel programming to handle data exchange and matrix operations in the construction and diagonalization of large-scale Hamiltonian matrix within plane waves. Numerical results illustrate that these iterative eigensolvers can scale up to 42,592 processing cores with high peak performance of 30% on leadship supercomputers to study the electronic structures of bulk silicon systems containing 10,648 atoms. Program summaryProgram Title: Plane wave density functional theory (PWDFT)CPC Library link to program files:https://doi.org/10.17632/c8v2mx5vn4.1Developer's repository link:https://bitbucket.org/berkeleylab/scalesLicensing provisions: BSD 3-clauseProgramming language: C++Nature of problem: PWDFT is used for electronic structure calculations based on Kohn-Sham density functional theory. The key challenge to address is a constrained energy minimization problem, which can also be formulated as a nonlinear eigenvalue problem. MPI/OpenMP-based approaches are employed to provide multi-core acceleration for the study of the chemical and material properties of larger-scale molecules and solids.Solution method: PWDFT implements self-consistent field (SCF) iterations and direct constrained minimization algorithms with various acceleration strategies. It is written in C++ and offers parallel acceleration based on MPI/OpenMP.

Polarization characteristics and structural modifications of Cu nanoparticles under high electric fields

High electric fields affect the diffusion dynamics of atoms on a metal surface, causing biased surface diffusion that possibly leads to the growth of intensively field emitting protrusions and consequent vacuum breakdown (VBD). The scientific understanding of this process, as well as other fundamental VBD initiation mechanisms, is far from complete. Here we investigate the exact atomic behaviour of metal surfaces exposed to extremely high electric fields using density functional theory (DFT). Previous theories describe the field-surface dynamics in terms of the effective dipole moments and polarizability of surface atoms, disregarding higher-order (hyperpolarizability) terms. The validity of this approximation has been evaluated only for electric fields up to 3 GV/m, due to computational limitations of the plane-wave DFT basis used in previous works. In this work, we test the validity of this approximation for a much wider field range, relevant for VBD and field emission (FE), using Cu nanoparticles as our test structures. We find that although such high fields can change the entire structure of Cu nanoparticles, their energetics are described very precisely by the permanent dipole moment and polarizability terms. Thus, we show that neglecting the hyperpolarizability terms is valid even for field values that exceeds the range that is relevant for intense FE and VBD. This work lays a solid foundation for further developing atomic-level simulation models for electric field-induced surface diffusion on metal surfaces and its effects on protrusion growth and VBD initiation.

Development and application of hybrid AIMD/cDFT simulations for atomic-to-mesoscale chemistry.

Many important chemical processes involve reactivity and dynamics in complex solutions. Gaining a fundamental understanding of these reaction mechanisms is a challenging goal that requires advanced computational and experimental approaches. However, important techniques such as molecular simulation have limitations in terms of scales of time, length, and system complexity. Furthermore, among the currently available solvation models, there are very few designed to describe the interaction between the molecular scale and the mesoscale. To help address this challenge, here, we establish a novel hybrid approach that couples first-principles plane-wave density functional theory with classical density functional theory (cDFT). In this approach, a region of interest described by abinitio molecular dynamics (AIMD) interacts with the surrounding medium described using cDFT to arrive at a self-consistent ground state. cDFT is a robust but efficient mesoscopic approach to accurate thermodynamics of bulk electrolyte solutions over a wide concentration range (up to 2M concentrations). Benchmarking against commonly used continuum models of solvation, such as SMD, as well as experiments, demonstrates that our hybrid AIMD-cDFT method is able to produce reasonable solvation energies for a variety of molecules and ions. With this model, we also examined the solvent effects on a prototype SN2 reaction of the nucleophilic attack of a chloride ion on methyl chloride in the solution. The resulting reaction pathway profile and the solution phase barrier agree well with experiment, showing that our AIMD/cDFT hybrid approach can provide insight into the specific role of the solvent on the reaction coordinate.

Effects of -H and -OH Termination on Adhesion of Si-Si Contacts Examined Using Molecular Dynamics and Density Functional Theory.

The contact between nanoscale single-crystal silicon asperities and substrates terminated with -H and -OH functional groups is simulated using reactive molecular dynamics (MD). Consistent with previous MD simulations for self-mated surfaces with -H terminations only, adhesion is found to be low at full adsorbate coverages, be it self-mated coverages of mixtures of -H and -OH groups, or just -OH groups. As the coverage reduces, adhesion increases markedly, by factors of ∼5 and ∼6 for -H-terminated surfaces and -OH-terminated surfaces, respectively, and is due to the formation of covalent Si-Si bonds; for -OH-terminated surfaces, some interfacial Si-O-Si bonds are also formed. Thus, covalent linkages need to be broken upon separation of the tip and substrate. In contrast, replacing-Hgroups with -OH groups while maintaining complete coverage leads to negligible increases in adhesion. This indicates that increases in adhesion require unsaturated sites. Furthermore, plane-wave density functional theory (DFT) calculations were performed to investigate the energetics of two Si(111) surfaces fully terminated by either -H or -OH groups. Importantly for the adhesion results, both DFT and MD calculations predict the correct trends for the relative bond strengths: Si-O > Si-H > Si-Si. This work supports the contention that prior experimental work observing strong increases in adhesion after sliding Si-Si nanoasperities over each other is due to sliding-induced removal of passivating species on the Si surfaces.

FIRST-PRINCIPLES CALCULATION OF STRUCTURAL, ELECTRONIC, AND OPTICAL PROPERTIES OF InP1-XSbX USING WC-mBJ FOR NANOSCALE IR APPLICATIONS

The structural, electronic, and optical characteristics of cubic InP<sub>1-x</sub>Sb<sub>x</sub>(x = 0, 0.25, 0.50, 0.75, 1) ternary alloys were explored using the full-potential linearized augmented plane wave density functional theory approach. The total energy vs. volume optimization, lattice constants, and density of states were investigated for InP<sub>1-x</sub>Sb<sub>x</sub> alloys using exchange correlation function Wu-Cohen generalized gradient approximation (WC-GGA), available with the WIEN2k code. Band structure of the alloys was calculated using TB-mBJ functional to achieve a higher bandgap accuracy. The results of the mBJ experiment are in close agreement to those of the other experimental studies when compared to WC-GGA. Dielectric function and energy loss function were calculated in order to explore optical properties of the alloys. It was noticed that the estimated lattice parameters exhibit reduction when the Sb content is increased. Furthermore, the compositional dependency of the structural, electronic, and optical properties were also reported. For the InP<sub>1-x</sub>Sb<sub>x</sub> alloys, a band gap of less than 1.6 eVwere observed, making it suitable for usage in infrared optoelectronics devices.

CO2 ultrathin film growth on a monolayer of CO2 adsorbed on the NaCl(100) surface: sticking coefficient and IR-optical signatures in the ν3 region.

CO2 ultrathin molecular films were grown onto a preadsorbed monolayer NaCl(100)/p(2 × 1)-CO2 at 40 K. Polarization infrared spectroscopy (PIRS) reveals that so-prepared films have better quality than directly grown films. A sticking probability of 0.74 ± 0.1 was deduced from the integrated IR absorption. The presence of the monolayer doublet in the film spectra suggests a Stranski-Krastanov film growth with locally varying film thicknesses on the surface. In the region of the ν3(12C16O2) band, fine structure was observed between the well-known transverse-optical (TO) and longitudinal optical (LO) bands. Two independent computational models were applied to analyze the nature of the observed fine structure. Both pair potential calculations in combination with a vibrational exciton model as well as plane-wave density functional theory (DFT) in combination with phonon calculations of IR intensities at the Γ-point reveal that a weak mode visible in s-polarization and p-polarization originates from a vibrational film excitation located near the substrate interface. A series of p-polarized weak bands appearing and partly disappearing upon film-growth is assigned to film stacks of unique local thickness.

Theoretical study of thermal stability of α″–Fe16N2 against other iron nitrides

α″–Fe16N2 has been investigated as one of promising candidates for environment-friendly magnets. While giant saturation magnetization has previously been experimentally observed in α″–Fe16N2, its magnetic anisotropy and structural stability leave room for improvement. Recent theoretical studies have considered alloying Fe16N2 with various elements to improve the magnetic properties and/or stability against decomposition. However, estimates of stability in particular are typically restricted to simple ground-state-energy comparisons, i.e. effectively taken at 0 K. For a more practical measure of stability, we therefore extend ground-state energies, obtained with the plane-wave density-functional theory code Quantum ESPRESSO, with appropriate empirical and/or statistical corrections to obtain free energies at arbitrary temperature. We then compare the stability of Fe16N2 against the neighboring phases in the Fe-N binary system, to estimate the range of temperatures at which it is stable. We compare against experimental observations of the Fe-N phase diagram.

An implicit electrolyte model for plane wave density functional theory exhibiting nonlinear response and a nonlocal cavity definition.

We have developed and implemented an implicit electrolyte model in the Vienna Ab initio Simulation Package (VASP) that includes nonlinear dielectric and ionic responses as well as a nonlocal definition of the cavities defining the spatial regions where these responses can occur. The implementation into the existing VASPsol code is numerically efficient and exhibits robust convergence, requiring computational effort only slightly higher than the original linear polarizable continuum model. The nonlinear + nonlocal model is able to reproduce the characteristic "double hump" shape observed experimentally for the differential capacitance of an electrified metal interface while preventing "leakage" of the electrolyte into regions of space too small to contain a single water molecule or solvated ion. The model also gives a reasonable prediction of molecular solvation free energies as well as the self-ionization free energy of water and the absolute electron chemical potential of the standard hydrogen electrode. All of this, combined with the additional ability to run constant potential density functional theory calculations, should enable the routine computation of activation barriers for electrocatalytic processes.

Adsorption of 4-(N,N-Dimethylamino)-4'-nitrostilbene on an Amorphous Silica Glass Surface.

Stilbenes are a compelling class of organic photoswitches with a high degree of tunability that sensitively depend on their environment. In this study, we investigate the adsorption properties of 4-(N,N-dimethylamino)-4'-nitrostilbene (DANS), a push-pull stilbene, on amorphous silica glass. Plane-wave density functional theory (DFT) calculations are used to understand how the trans and cis isomers of DANS interact with the amorphous surface and which are the most preferred modes of adsorption. Our calculations revealed that the O-H···O hydrogen bonds between the nitro group and hydroxyl groups of the silica surface dominate the intramolecular interaction. In addition to hydrogen bonding, O-H···π interactions with the aromatic ring and double bond play a critical role in adsorption, whereas C-H···O interactions are present, but contribute little. Therefore, both isomers of DANS favor parallel orientations such that not only the functional groups but also the aromatic parts can strongly interact with the glass surface.

Theoretical study of the photocatalytic activity of hBNC/MoSX (X = Se, Te) heterojunctions

The effects of the introduction of C-C bonds on the stability, electronic structure, work function, and optical properties of hBN/MoSX heterojunctions are investigated based on the density functional theory plane wave ultra soft pseudopotential method. The results show that the hBNC/MoSX heterojunctions have low binding energies (−4.066 eV to −4.777 eV) and lattice mismatch ratios (2.62 % and 1.44 %) and can be stabilized at room temperature of 300 K, indicating that they possess excellent stability. By comparing the electronic structure with that of the hBN/MoSX heterojunction, it is found that the introduction of C-C bonds can effectively adjust the band gap of the heterojunction. The bandgap of hBN/MoSX varies from 1.018 eV to 1.168 eV with small variations. The hBNC/SMoSe and hBNC/SMoTe heterojunctions have bandgaps of 0.309 eV and 0.494 eV, with significantly reduced bandgaps and orbital hybridization. The number of electrons in the excited state increases, which makes the electron leap easier and is beneficial to improve the response of the heterojunction to visible light. Due to the presence of the built-in electric field at the interface, the compounding of photogenerated electron-hole pairs can be effectively suppressed, which greatly enhances both the heterojunction photocatalytic ability and carrier migration. Compared with the hBNC monolayer system, both were red-shifted in the low-energy region. hBNC/SMoTe and hBNC/SMoSe showed the best absorption of light in the low-energy region, indicating excellent photocatalytic performance.

First principle study of enhanced CO adsorption on divacancy graphene-supported TM7 (TM = Fe, Co, Ni, Cu, Ag, and Au) clusters

The adsorption of CO on transition metal clusters TM7 (TM = Fe, Co, Ni, Cu, Ag, and Au) and their corresponding divacancy graphene-supported clusters has been investigated using plane-wave density functional theory with van der Waals D3 correction. The results reveal that all divacancy graphene-supported TM7 clusters show a stronger adsorption capacity for CO compared to the pure TM7 clusters. Orbital-resolved bonding interactions indicate, while the d-p orbital coupling between the substrate and CO dominates the magnitude of the absolute adsorption energy, the enhanced s-p orbital coupling between the substrate and CO determines the increase in the adsorption energy of CO on the divacancy graphene-supported TM7 clusters. On the other hand, the enhanced CO chemisorption energies on the divacancy graphene-supported TM7 clusters are caused by the induced electrostatic interaction between the positively charged TM7 on divacancy graphene and CO.

Elastic, electronic, and magnetic properties of MNb2O6 (M = Mn, Fe, Co, Ni) in the orthorhombic Pbcn structure, a computational study

The orthorhombic niobates containing magnetic transition ions such as Mn, Fe, Co, and Ni show excellent promise for materials in energy storage, electro-photoactivity, and low-dimensional magnetic character. We report on the computational studies focusing on the elastic, electronic, and magnetic properties of four compounds with chemical formulae MNb2O6 (M = Mn, Fe, Co, Ni) in orthorhombic Pbcn crystal structure using the plane wave density functional theory (DFT) computations. The fully optimized crystal structures were used to study the elastic stiffness, compliance, bulk, and young moduli, the Poisson ratios, and anisotropy constants in both single crystal and polycrystalline compounds. Moreover, the electronic structure including the bandgaps, density of states, and magnetic states was also determined. The bandgap values of 2.98 eV, 2.64 eV, 2.70 eV, and 1.86 eV were obtained for MNb2O6 (M = Mn, Fe, Co, Ni), respectively. The Young’s modulus in all samples showed anisotropic behavior. The sound velocity and Debye’s temperature showed a decrease across the series MNb2O6 (M = Mn, Fe, Co, Ni). All four compounds have an antiferromagnetic ground state and the spin-only moments of these ions were found to be 4.55 μB for Mn3+, 3.62 μB for Fe3+, 2.64 μB for Co3+, and 1.68 μB for Ni3+. Overall, this study provides a comprehensive theoretical investigation of the basic materials properties of MNb2O6 (M = Mn, Fe, Co, Ni) which will enable the researcher working on the application of these materials in devices to carefully tailor their properties according to the application requirements.

Competitive adsorption between sodium citrate and naphthenic acids on alumina surfaces: Experimental and computational study

The adsorption of naphthenic acids (NAs) on clay surfaces induces an increase in surface hydrophobicity, which is detrimental to crude oil recovery and froth quality. Sodium citrate (Na3Cit), with more functional groups than NAs, is expected to compete with NAs for adsorption on mineral surfaces. The adsorption of NAs on alumina surfaces was quantified in the presence and absence of Na3Cit at pH 8 ± 0.1. An increase in hydrophobicity of the alumina surface was observed after the adsorption of NAs without Na3Cit. However, adding Na3Cit significantly reduced NAs adsorption and protected the surface wettability. The viscoelasticity and electrokinetic properties of adsorbed materials indicate Na3Cit was preferentially covered on alumina surfaces and repelled NAs by its negative charges. The competition between Na3Cit and NAs for adsorption sites was explained using ab initio Car-Parrinello molecular dynamics (CPMD) simulation and periodic plane-wave density functional theory (DFT) to calculate inner-sphere adsorption and outer-sphere adsorption, respectively. Favourable adsorption of sodium citrate than NAs representatives on aluminum (hydr)oxide cluster and the basal and edge planes of gibbsite were observed from the perspective of free energy barrier and adsorption energy, respectively.