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Thermodynamic stability is regularly quantified using the enthalpy of formation () and the Gibbs free energy of formation (ΔfH). Although knowledge of these properties is crucial for many applications, literature data are often missing for many solids. We evaluate the harmonic approximation (HA) for the prediction of ΔfH and ΔfGat constant volume and pressure, based on phonon calculations. Using density functional theory to carry out phonon calculations, we show that the HA excellently describes the temperature-dependence of ΔfH and ΔgH for 14 compounds; mean absolute error (MAE) of 2.1 kJmol−1 and 1.1 kJmol−1, respectively, in the temperature interval 0–2500 K. Moreover, the performance of the HA was evaluated using computational data from the Materials Project database for 69 compounds yielding an MAE of 1.1 kJmol−1 and 1.1 kJmol−1, respectively, in the temperature interval 100–800 K. Very good performance is also observed for a number of additional compounds, including several hydrated salts, at 298.15 K. The model is subsequently applied to a number of phase-transition phenomena that demonstrate the strengths and weaknesses of the model. In addition, it is demonstrated that the HA model can provide quantitative performance that rivals that of the conventional quasi-harmonic approximation for the prediction of formation properties, at a significantly reduced computational effort (∼5–10 times faster).

In this work, the role of surface oxygen in the early stage of graphene growth is investigated using density functional theory (DFT) and first-principles molecular dynamics (FPMD) simulation. For the complete dehydrogenation of CH4 on a bare or oxygen-pre-adsorbed Cu(1 1 1) surface, reaction pathways and energy barriers are compared using the Climbing Image Nudged Elastic Band method (CI-NEB). Then, the dynamics of surface oxygen with regard to graphene growth is discussed by analyzing the behavior of CH fragments on an oxygen-pre-adsorbed Cu(1 1 1) surface for two different temperatures using FPMD. After analyzing the effect of oxygen on carbon feedstock dissociation and aggregation in the context of graphene growth, it can be concluded that surface oxygen is a decisive factor during oxidation, dehydrogenation, and carbon movement. It facilitates both reduced graphene nucleation density and film coverage, as well as a high growth rate for graphene.

Using density functional theory calculations, the electronic structure and transport properties of borophene, and also the stable structures of hydrogenated and fluorinated borophene are investigated. The structural properties, projected density of states, charge transfer, band structure, Fermi velocity, current-voltage characteristic and transmission spectrum of borophene and other considered structures are studied and compared. The results indicate that borophene, single side fully hydrogenated borophene, single side partially hydrogenated borophene, and fluorinated borophene have metallic characteristics, while double side fully hydrogenated borophene is a semimetal with a linear dispersion relation at low energies. Furthermore, the current-voltage characteristics of all studied structures show Ohmic behavior. The findings demonstrate that boron atoms of hydrogenated borophene act as electron acceptors while in fluorinated borophone they have a donor characteristic. High Fermi velocity which appears in double side fully hydrogenated borophene makes it highly applicable in nanoelectronic devices in the future. Moreover, the results reveal that saturation types can affect observed anisotropic behavior in the electronic transport properties of borophene so it can be suitable to design functional devices such as switches.

Bimetallic selenocyanate complex ZnCd(SeCN)4 (ZCSC) crystals are a novel material with ultraviolet nonlinear optical (NLO) properties. To the best of our knowledge, theoretical studies on these complexes have mostly focused on the microcosmic NLO responses of single complex molecular system and ignored their macroscopic NLO effects. In this paper, the geometric structure of ZCSC crystal was optimized on the basis of first principle, and the electronic band structure and linear optical (LO) properties were investigated via pseudopotential plane wave method. The macroscopic NLO properties of ZCSC crystal were evaluated through the combination of density functional theory and a devised anionic group method for the first time, and good agreement with the experimental results was obtained. The absorption spectrum of ZCSC was located at 95–325 nm, and the NLO properties were mainly caused by the transitions of electronic states from N2p and Se3p valence bands to C2p, N2p, Se3p, and Cd4p conduction bands. Results showed that the second-order NLO coefficients were d14 = 2.304–3.55 pm/V and d15 = 8.07–9.91 pm/V, showing an evident increase compared with those of ZnCd(SCN)4 crystals.

By combining the density functional theory and nonequilibrium Green’s function, we have studied the electronic, optical and transport properties of multilayer two-dimensional (2D) semiconducting MXenes. Firstly, the band gaps of multilayer MXenes are investigated under an external transverse electric field. Our results show that the band gaps slightly decrease with increasing of number of layers and the increasing electrical field strength. Next, the layer number dependent real and imaginary parts of the dielectric functions for these semiconducting MXenes are predicted, which indicates the larger absorption in both the visible and ultraviolet spectral ranges. Lastly, the transport properties of multilayer M2CO2 (M = Ti, Zr, or Hf) p-n devices are studied. The Ti2CO2 p-n devices show higher conductance than the others, and a typical behavior for conventional diodes of all systems are also clearly observed.

First principles density functional theory calculations were performed to understand the electronic and atomistic factors governing adhesion and resistance to shear for simple metal/ceramic interfaces. The work of adhesion and two-dimensional generalized stacking fault energies were calculated and examined for the 9 interfaces resulting from the combination of three metals (Cu, Ti, and Cr) with three ceramics (TiN, VN, and CrN). It was found that minimizing the lattice mismatch between metal and ceramic was an important factor for maximizing adhesion and barriers to shear, especially a few metal layers away from the chemical interface. Electron or charge density was found to be a good descriptor of shear barrier heights between metal layers (typical of shear displacement away from the chemical interface), while not very good for nitrogen-metal bonds (typical of the chemical interface). Cu/ceramic interfaces in particular were more stable when they were in semi-coherent interfacial configurations, but the barriers to shear were usually smaller than for coherent configurations, which is attributed to the complex nature of atomic interfacial shear movement resulting from the greater diversity of atomic environments and interactions.

Adsorption in porous media has a direct impact on the reserves of fluid in shale gas reservoirs, but they are rarely studied in combination with molecular simulation and quantum mechanisms. The adsorption behaviors between methane, carbon dioxide and quartz are investigated with Grand Canonical Monte Carlo, Molecular Dynamics and Density Functional Theory in this work. Adsorption isotherm, density distribution, adsorption energy and Density of States (DOS) are calculated and analyzed. These results are as below. The adsorption isotherms of methane, carbon dioxide and their mixture (mole ratio 1:1) can be described as Langmuir type. The adsorption amounts increase with the rising of pressure, and the adsorption capacity of carbon dioxide is stronger than methane. The position near the wall is occupied by methane and carbon dioxide, and density distribution shows that the thickness of adsorption layer is 5 A. The distribution of adsorption energy states clearly physical adsorption. DOS of methane systems have a little bit difference, and a relatively large difference of peak value is in carbon dioxide systems. DOS of adsorbents (maximum and minimum adsorption energy) are basically coincided with each other, while DOS of methane and carbon dioxide shift with different degrees to the lower energy region.

Cycloparaphenylene (CPP) crystals comprising of molecular carbon nanorings are a unique class of carbon-based nanomaterials possessing inimitable properties. Based on molecular dynamics simulations and density functional theory calculations, the mechanical and thermal properties of CPP crystals are investigated in the present study. Our results show that CPP crystals possess an ultrahigh energy absorption capacity, an ultralow thermal conductivity and an ultralight weight simultaneously, which can be attributed to their intrinsically porous lattice structures and also the nanoscale dimension of their component nanoring cells. Such unique coexistence of high energy absorption and thermal insulation capacities in CPP crystals makes them more appealing in some extreme environment applications when compared to most existing engineering materials. Moreover, the energy absorption capacity and thermal conductivity of CPP crystals are also found to strongly depend on the cell size and packing mode of their component CPP molecules. The high tunability of the energy absorption capacity and thermal conductivity observed in CPP crystals raises the potential to design them for specific application requirements.

Akio Ishii, Activation energy of homogeneous nucleation of Zr hydride: Density functional theory calculation, Computational Materials Science, Volume 215, 2022, 111769, ISSN 0927-0256, https://doi.org/10.1016/j.commatsci.2022.111769., Considering the nucleation process of Zr hydrides as phase transformation from hexagonal closed-packed (HCP) to face-centered tetragonal (FCT) structure, we calculated the activation energy of the homogeneous nucleation process of Zr hydrides and atomic rearrangement during nucleation for Zr4H, Zr2H, ZrH and ZrH2 using density functional theory calculations and minimum energy path detection. At 0 K limit, although ZrH and ZrH2 have lower chemical potentials and are more energetically stable than Zr4H and Zr2H, the latter have lower activation energies for nucleation. At finite temperatures, the crossover of activation energies occurs around 300 K, where ZrH becomes the most possible candidate with the lowest activation energy. This was explained by the difference in the atomic rearrangement and change in phonon frequency during phase transformation.

Aragonite (CaCO3, space group Pmcn) is an important mineral for both geological and biological reasons, being one of the phases that recycles carbon in deep Earth conditions and the product of biomineralization of several terrestrial and marine organisms, respectively. Because of its ubiquity, aragonite has been the subject of several investigations to understand its elastic behaviour and stability at different P-T conditions, but the results reported in literature are still very scattered. Aiming at providing further details on this topic, in the present work we determined the structural and elastic properties of aragonite at absolute zero (0 K) within the Density Functional Theory framework, using a posteriori correction to include the weak long-range interactions. The equation of state parameters for this mineral phase, calculated between 0 GPa – 25 GPa, were K0 = 80.2(7) GPa, K’ = 4.37(10) and V0 = 223.00(6) Å3, in good agreement with the bulk modulus calculated from the elastic moduli (KR = 78.49 GPa). The results were compared to previous experimental and theoretical data, finding them in line with some specific studies, and show that some structural features (e.g., the carbonate ion aplanarity) could be related to the mechanism of phase transition to the post-aragonite phase at high pressure. The present work highlights the importance of including van der Waals interactions in the physical treatment of the structural and elastic properties of aragonite, and further extends the knowledge of the behaviour of this mineral as a function of pressure.

There has been a debate on the theoretical explaining of the spin states in the perovskite ACoO 3 , especially SrCoO 3 . The aim of this work was to investigate the magnetic phase stability and to study the evolution of the spin states of the cubic perovskite ACoO 3 (A = Ca, Sr) under pressure up to 40 GPa by using the density functional theory and the Hubbard +U parameters (DFT+U). Both CaCoO 3 and SrCoO 3 exhibit the ferromagnetic and metallic properties under pressure. Our results of the total magnetization and the net-spin density corroborate that a change of the magnetic behaviors on ACoO 3 mostly depends on the value of Co-O bond length ( d C o − O ). We discovered a down-stepwise evolution of the total magnetization as a relationship with the d C o − O decreasing. This stepwise behavior occurs prior to the spin crossover from intermediate spin (IS) state to low spin (LS) state. The total magnetization values non-monotonically decreased from 2.49 to 2.16 μ B/f.u. between the d C o − O = 1.88 and 1.80 A. From our evidence, we indicated that the spin state of Co- 3 d states resembles the d 7 L 2 character with t 2 g 3 ↑ , 2 ↓ e g 2 − x ↑ , x ↓ L 2 state, where x ≈ 0 . 5 and L denotes the oxygen’s charge transfer from the oxygen ligands. Indeed, we discovered that the stepwise evolution of the total magnetization is associated with the changes in the band topology of the Co- 3 d e g ∗ band.

The structural, lattice dynamical, and electronic properties of non-stoichiometric Cd-Se binary compounds under high pressures were investigated by using a combined approach of particle swarm optimization (PSO) structure search and density functional theory (DFT) calculations. A novel phase of CdSe4, which is stable under 50 and 100 GPa, is identified through thermodynamic and lattice kinetic calculations. Unlike the tetrahedrally coordination in wurtzite and zinc-blende CdSe compounds, CdSe4 possesses stronger anharmonic interactions and is featured by its Cd-centered and Se-centered octahedral building blocks. The central Cd atom adopts equivalent d2sp3 hybridization and forms six equivalent bonds with surround Se atoms. Se atoms not only bond with Cd atoms, but also form bonds with each other. Moreover, band structure and electron localization function analysis reveals that CdSe4 exhibits metallicity instead of semiconductivity, resulting from its metallic bonding between Se atoms. Our study enriches the II–VI chemistry and provides further understanding to the semiconducting and conducting properties of Cd-Se binary compounds.

Predictions of the anisotropic coefficients of thermal expansion are needed to not only compare to experimental measurement, but also as input for macroscopic modeling of devices which operate over a large temperature range. While most current methods are limited to isotropic systems within the quasiharmonic approximation, our method uses first-principles calculations and includes anharmonic effects to determine the temperature-dependent properties of materials. These include the lattice parameters, anisotropic coefficients of thermal expansion, isotherm bulk modulus, and specific heat at constant pressure. Our method has been tested on two compounds (Cu and AlN) and predicts thermal properties which compare favorably to experimental measurement over a wide temperature range.

Ni and its alloys are susceptible to hydrogen embrittlement (HE). In this work, we perform a systematic density functional theory (DFT)-based investigation on the hydrogen-enhanced decohesion (HEDE) mechanism of HE for the case of the special Σ 5(0 1 2) grain boundary (GB) in Ni containing C and Mo impurity atoms. Segregation and co-segregation energy profiles of H along with C and Mo solute elements are investigated in detail and used to analyze the effect of Mo and C solutes on HEDE in Ni within the framework of the Rice-Thomson-Wang theory. We show that H, C, and Mo segregate to the GB in Ni. H demonstrates the GB embrittling effect while C and Mo solutes strengthen the GB in Ni. The results also show that H–Mo and H–C interactions in the bulk and at the GB are very similar and can be neglected in most of cases of co-segregation.

Using first-principles calculations based the density functional theory, we investigate the electronic structures and superconducting properties of iron-based superconducting films MFeAs (M = Li/Na) under the different strains. The calculations show that when the 1–5% compressive and tensile strains are applied, although the striped antiferromagnetic ground state of the MFeAs films remain unvaried, the Fermi surface nesting between electron-hole pockets, the electronic density of state at Fermi energy and magnetic moments of Fe ions all undergo the significant variations. The applications of compressive strains improve the Fermi surface nesting, decrease the localized magnetic moments of Fe ions, and increase the electronic density of states at the Fermi level of films. The variations suggest that the compressive strains favour the superconductivity by facilitating the spin fluctuations in films. However, the energy bands and electronic properties of MFeAs films under tensile strains show the opposite variations, and tend to suppress the superconductivity of films. Our results provide a simple solution and a comprehensive explanation for the improved superconductivity of MFeAs films by compressive strains.

Ionic conduction in LaOF was investigated by density functional theory-based first-principles calculations. The origin and microscopic mechanism of selective fluoride ion conduction in LaOF were investigated by evaluating the formation energy of the Frenkel pair and by performing ab initio molecular dynamics (MD) simulations for both tetragonal (anion order along the [0 0 1] direction) and rhombohedral (anion order along the [1 1 1] direction) structures experimentally obtained in LaO1−xF1+2x. In both structures, the formation of an F Frenkel pair was energetically favored over that of an O Frenkel pair, and the F Frenkel pair predominantly contributed to the conduction of fluoride ions. From the trajectory of MD simulations with an F or O Frenkel pair in the two structures, it was found that fluoride ion conduction mainly occurred via an interstitialcy mechanism. This fluoride ion migration occurred only in the tetragonal structure along the (0 0 1) plane. This observation was further confirmed by migration barriers evaluated by the climbing-image nudged elastic band method. The results suggested the possibility of the enhancement of ionic conductivity by controlling the anion ordering in mixed anion compounds.

Li-doping and functionalization are two approaches to enhance the hydrogen adsorption capacity of metal organic frameworks (MOFs). Herein, we report the simulations of modified MOF-519 implemented by Grand canonical Monte Carlo (GCMC) methodology and Density Functional Theory (DFT) to evaluate the degree of enhancement of hydrogen absorption. The adsorption mechanism and preferred adsorption sites are also investigated. The GCMC results show that Li can significantly improve hydrogen storage capacity. Besides, all functionalized MOF-519-X (X = OH, NO2, Cl, NH2, CH3, F) figure better adsorption performance. Compared to parent MOF-519, Li-MOF-519 has a gravimetric adsorption of 2.265 wt%, and MOF-519-OH performs the highest gravimetric adsorption among the functionalized MOFs with 2.362 wt%. Furthermore, the enhancement on hydrogen adsorption is also studied from the aspect of volumetric adsorption and isosteric heat.

There has been increasing interest in organic semiconducting polymers for use in organic electronics due to their reasonable charge mobility, low-cost and environmental stability. In this work, bithiophene-imide (BTI) comonomers are designed systematically in silico by combining the electron acceptor BTI unit with different types and numbers of interior thiophene units as electron donor groups. The effects of functionalization of BTI unit on the electronic, optical and charge mobility properties of comonomers are investigated by density functional theory (DFT) calculations. The presence of nitro substitution on the BTI unit lowers the optical band gaps and results in high hole mobilities. The copolymer-6 is estimated to be the best p-type organic semiconductor among all studied copolymers with the highest hole mobility of 1.31 cm2 V−1 s−1. The adsorption isotherms of the copolymers for the H2O, CO2 and O2 adsorbate molecules are analyzed by Monte Carlo simulations to elucidate the air and water stabilities for better device performances.

By means of density functional theory (DFT) calculations, we investigate the fluorination and hydrogenation activities of h-BN layer and the surface frictional properties of fluorinated h-BN on commensurate Ni(1 1 1) substrate. The calculation results indicate that Ni(1 1 1) influences the chemical activity of h-BN and the F atoms and H atoms prefer to adsorb on the B atoms and N atoms, respectively. The fluorination activity on B atoms is largely enhanced, while the hydrogenation on h-BN layer is much difficult. Meanwhile commensurate Ni(1 1 1) confines the topography corrugation caused by fluorination and the frictional properties of flattened fluorinated h-BN on Ni(1 1 1) are investigated. It is found that the flattened surface of fluorinated h-BN maintains the potential energy corrugation when sliding against a single h-BN layer, and the sliding potential energy corrugation decreases remarkably for the paired fluorinated surfaces. Variations of binding energy and friction at the interface after chemical modification are understood by interfacial charge redistribution. This study indicates the low friction of h-BN can be preserved in the presence of a commensurate Ni(1 1 1) substrate under chemical modification environment.

The identification of natural mineral photocatalysts with excellent visible-light absorption, suitable band edge positions, and large carrier mobility is significant for water-splitting applications. Herein, by applying density functional theory simulation, the natural sulvanite Cu3MX4 (M = Nb, Ta; X = S, Se) compounds are proposed as efficient photocatalysts for visible-light water splitting. Calculation of the phonon dispersions and elastic constants of the Cu3MX4 (M = Nb, Ta; X = S, Se) compounds reveal that they are dynamically stable. In addition, their electronic structures indicate that these compounds are indirect semiconductors with band gaps in the desired range of 2–3 eV, and the position of the band-edge energy perfectly straddles the water oxidation and reduction potentials. Partial density charge analysis reveals that the electrons are mainly located near the Nb/Ta S/Se bond, whereas the holes occupy the Cu S/Se bonds. Furthermore, the effective mass, deformation potential parameters, and carrier mobility were quantitatively investigated to evaluate the photocatalytic dynamics. A small average effective mass and effective separation of photogenerated carriers in space was observed. Our calculations provide guidance for experimental synthesis and will expand the scope of these natural minerals for visible-light-driven water splitting with excellent performance.

By using the density functional theory, we propose that the two-dimensional (2D) SiGe is a promising anode material of a sodium or potassium ion battery. We confirm the thermal and dynamic stabilities of the SiGe sheet by calculating the formation energy and phonon dispersion, respectively. The SiGe sheet provides moderate/low migration energy barriers for the alkali metal atoms (0.14–0.35 eV), suggesting fast charge/discharge rates. The SiGe sheet gives high theoretical capacities (for Li/K ∼ 532 mA h g−1 and for Na ∼ 1064 mA h g−1) and stable voltage profiles.

The generalized stacking fault energies of the {0001}〈10–10〉, {1–100}〈11–20〉 and {11–22}〈11–23〉 slip systems and surface energies of {0001} surface are investigated on pure Mg, Mg-Zn and Mg-Zn-R (R = La to Sm) models, through first-principles calculations based on density functional theory. The results show that the addition of alloying element R enhances the intrinsic ductility of Mg-Zn alloys. Moreover, the effect of solute concentration on surface stability is examined by Mg-Zn-La model, and the result suggests that the surface becomes more stable with the decrease of solute concentration. Thereby, the formability and surface stability of Mg-Zn alloys can be modified by adding rare-earth elements R.

The stability, band structures and electronic properties of the γ-graphdiyne nanoribbons with half of the hexagonal rings at edges being cut off (hc-GDYNRs) are investigated by using the density functional theory. For the armchair hc-GDYNRs, the ground state is the nonmagnetic state with a direct bandgap. However, the variations in the bandgap and the mobility are both very different from those of the corresponding γ-graphdiyne nanoribbons with intact hexagonal rings at edges (GDYNRs). The electron mobility of up to 106 cm2 V-1 s−1 may enable the armchair hc-GDYNRs to be potential candidates for high-speed electronic devices. For these wider zigzag hc-GDYNRs, the two frontier bands form nearly degenerate flat-bands at the Fermi level in the nonmagnetic state. This is mainly due to the localized distribution of the crystal orbitals at the edge sites. Spin-polarization calculations indicate that for zigzag hc-GDYNRs, the antiferromagnetic state with a small bandgap (0.156–0.245 eV) is the ground state. These are quite different from the case in the zigzag GDYNRs. This significant difference manifests the great impact on the electronic properties of the GDYNRs for the change of edge structures.

The hexagonal close-packed (hcp) e -martensite phase in steels nucleates from the γ -austenite parent phase and can undergo further transformation to the α ′ -martensite phase or exist as a metastable phase depending on temperature, mechanical loading, and alloy chemistry. The solute-dependent lattice parameters and elastic stiffness coefficients C ij of hcp Fe influence the mechanical properties of steels containing the e -martensite phase, as well as the martensitic transformations between the phases. We use density functional theory to calculate the lattice parameters and C ij of single-crystal hcp Fe as functions of solute concentration in the dilute limit for the substitutional solutes Al, B, Cu, Mn, and Si, and the octahedral interstitial solutes C and N. Our computationally efficient methodology separates the solute dependence of the C ij into lattice strain and chemical bonding contributions. The computed data can be used to estimate the effect of solutes on polycrystalline elastic moduli and the strain energy associated with martensitic transformations. The data can also serve as inputs to microstructure-based models of multiphase steels containing the e -martensite phase.

A selective sampling procedure is applied to reduce the number of density functional theory calculations needed to find energetically favorable grain boundary structures. The procedure is based on a machine learning algorithm involving a Gaussian process, and uses statistical modelling to map the energies of the all grain boundaries. Using the procedure, energetically favorable grain boundaries in BaZrO3 are identified with up to 85% lower computational cost than the brute force alternative of calculating all possible structures. Furthermore, our results suggest that using a grid size of 0.3 A in each dimension is sufficient when creating grain boundary structures using such sampling procedures.

The structural and electronic properties of bulk and ultrathin films of V 2 O 5 and MoO 3 layered oxides have been studied with first-principles density functional theory calculations including Van der Waals dispersion corrections. The U parameter in the DFT + U approach has been determined in order to properly reproduce geometry, band-gap, macroscopic dielectric constant, and formation enthalpies of the two materials. The mono-, and multi-layers are cleaved along the 〈0 0 1〉 and 〈0 1 0〉 stable crystallographic orientations for V 2 O 5 and MoO 3 , respectively. Three layers are needed in order to recover bulk-like properties of V 2 O 5 and MoO 3 . Spin-orbit effects have been incorporated in our simulations, and they result in marginal effects (∼20–30 meV) on the electronic band-gap of V 2 O 5 and a more pronounced (∼200 meV) effect on the band gap of bulk MoO 3 . We also discuss the importance of including local field effects for the reproduction of the anisotropy of the dielectric constant, which reflects the crystal structure of these materials

A crucial step in atomistic simulations of interfacial phenomena is confirming the thermodynamic consistency of the force field. This can be achieved by investigating the ability of the force field in predicting the bulk and surface properties of all components in the system. In this work, our focus is on the interfacial properties of gypsum, the calcium sulfate dihydrate (CaSO4·2H2O), which is the most abundant calcium sulfate in the earth crust. The performance of classical force fields was tested by comparing their results with those calculated with dispersion-corrected density functional theory (DFT-D) method as well as experimentally reported data. The DFT-D method implemented to study gypsum and showed a promising performance by reproducing experimentally reported bulk and surface properties. We showed that properties calculated using classical force fields are highly dependent on ionic partial charges. Substituting the original ionic partial charges in the CLAYFF force field with those of the INTERFACE force field improved the calculated bulk and surface properties significantly. CLAYFF with augmented partial charges and INTERFACE force fields showed the best overall performance in reproducing bulk and surface properties of gypsum.

The impact of machine learning (ML) methods is growing rapidly in materials science. One of the areas where we have shown promise is in the application of ML methods to sequentially guide experiments or computations towards targeted regions in the design space. Here, the need for ML methods is motivated by the fact that the search space is vast and complex. Further, it is intractable for the state-of-the-art high-throughput experimental or computational approaches to navigate the space in an efficient manner. In this review, two examples are discussed that demonstrate the efficacy of classification learning based ML methods in search for new and previously unexplored materials with desired properties. In the first study, ML methods guide density functional theory (DFT) phonon calculations in the design of Ruddlesden-Popper oxides with non-centrosymmetric (NCS) structures. Novel NCS compounds were predicted, whose thermodynamic stabilities were evaluated by DFT convex hull calculations. In the second study, ML methods enabled rapid screening of xBi[ Me y ′ Me ( 1 - y ) ″ ]O3–(1–x)PbTiO3 compositions in the desired perovskite crystal structure, where Me ′ and Me ″ are two octahedral site cations. The predictions were validated by experiments and new { Me ′ , Me ″ } cation pairs were identified that also have high ferroelectric Curie temperature.

Graphyne, a single atomic layer structure of the carbon six-member rings connected by one acetilenic linkage, is a promising anode of rechargeable batteries. In this paper, a first-principle study has been carried out on graphyne as a new candidate for the anode material of magnesium-ion batteries, using density functional theory calculations. The main focus is on the magnesium adsorption on graphyne surface. The structural properties such as adsorption height and energy, the most stable adsorption sites, the Band structure and DOS of the pristine graphyne the diverse Mg-decorated graphyne structures, and energy barrier against Mg diffusion are also calculated. As a consequence of the band structure and DOS of graphyne structures, it is found that the pristine graphyne and the Mg-decorated graphyne structures show a semiconducting nature and metallic behavior, respectively. Moreover, the migration behavior of Mg on graphyne for the main diffusion paths is determined by the Nudged Elastic Band (NEB) method., 7 pages, 5 figures

The multiferroic properties of the BiFeO3 (BFO) compound have received great attention for decades. To date, there are many detailed experimental and theoretical studies on the coexistence of ferroelectric (FE) and antiferromagnetic (AFM) orderings in the BFO which can show up a weak-ferromagnetism (w-FM) by canting of the ferromagnetic (FM) sublattice; however, there are only a few reports in the literature on BFO with FM ordering at the interfaces in heterostructures and, apparently, none on bulk FM BFO. Recently, a novel FM state coexisting with FE ordering at room temperature has been reported in (0 1 2)-oriented and strained BFO thin films [Ramirez-Camacho et al. Acta Materialia 128, 451–454 (2017)]. Based on such observed FM in BFO, the aim of our work is the theoretical study on the possibility of existence and stability of a macroscopic FM order coexisting with the FE arrangement in bulk BFO to explain the causes that lead to the observed FM-FE ordering. We outline the conditions that must be fulfilled by the crystalline structure, chemical bonding, and electronic structure employing the density functional theory (DFT) + Hubbard potential (U) formalism and using a combination of different representations as density of states, band structure, electron density, electron localization function, and molecular orbital. Our work demonstrates that the stable FM ordering is consequence of a small increase of the rhombohedral distortions and is strongly associated to higher degree of covalence of the spin-up eg orbitals in the Fe O bonds, and it is established by indirect exchange interactions such as superexchange and/or double-exchange interactions which are mediated by the oxygen ions located between the Fe ions. In particular, the electronic structure features of the possible FM order in the BFO compound suggest potential applications as voltage-controlled magnetic semiconductor for spin inverters in Spintronics.

We present a first principles study on the formation of Au nanowires in a break junction under different initial conditions. Density functional theory (DFT) was used to perform structural relaxations on four systems to investigate the effects of varying the number of relaxed layers and the system cross-section on the evolution of the break junction during the elongation process. The effect on the structure evolution by the increment value used for elongation was considered; therefore, structural relaxations with four selected increment values were performed separately. Electron transport calculations using the non-equilibrium Green’s function technique combined with DFT (NEGF-DFT) were also performed for each structure obtained from our relaxations. Even though the structures and junction stretching approaches were different, qualitatively similar results were found in all systems, including a single atomic chain and a conductance plateau with value of G 0 near the breaking point. We also found that the breaking distance and the final structure of the broken junction are different when either the number of relaxed layers or the increment value is changed.

Recently reported diphenylamine (DPA) in conjugation with biphenyl based D-π acceptor-D type mixed valence compounds (MV) have paved the way to design and development of new set of mixed valence compounds. In the present work, heteroaromatic π-acceptors (bi-furan, bi-thiophene and bi-pyrrole) have been used to design eight new molecules and their optoelectronic properties have been evaluated using density functional theory (DFT) calculations and compared with their biphenyl derivatives. Frontier molecular orbital analysis has been carried out to gain insights into role of different groups towards the HOMO and LUMO of the molecule. Optoelectronic properties such as absorption and emission spectra of DPA (neutral) molecules, mono- and di- cation spectra of all the DPA derivatives have been probed through time dependant density functional theory (TDDFT) calculations. All the newly studied MV compounds have a good electron delocalization, electron coupling constant and high redox potentials than their reported biphenyl derivatives. Furthermore, the study highlights a fine correlation between the computed optoelectronic properties and HOMO-LUMO energy gap, which portrays the role of hetero aromatic π-acceptors on the photo-physical properties of DPA derivatives.

The electron transport behavior through a series of symmetric and asymmetric dithiapentacenes (DPA) dithiolate junctions are theoretically investigated on the basis of density functional theory together with non-equilibrium Green’s function methods. The current-voltage characteristics of a series of DPAs with different separation of the heteroatoms have been calculated. The results indicate that the current correlates with the energy levels, the spatial distribution of molecular orbitals, and the transmission probability within the bias window. The separation of the two heteroatoms leads to an extension of the central quinoid structure, which forms the effective electron transport pathway. It can also lower the energy levels of the molecular orbitals and shift the transmission peaks towards the Fermi level, leading to a dramatic increase in the junction conductance. In particular, the current through 6,13-DPA is almost one hundred times greater than that through 5,12-DPA. Upon increasing the heteroatom separation, the junction current then tends towards a specific limit.

In-plane cleavage directions of two-dimensional (2D) crystals are displayed and often well defined in their flakes exfoliated by the mechanical exfoliation method. Here, we investigate the correlation between dominant in-plane cleavage direction and elastic properties in different directions for CrPS4. CrPS4 flakes show a preferential in-plane cleavage direction of 67.5°, corresponding to 〈1 1 0〉 direction. To explain it, we calculated the directional dependence of Young’s modulus and fracture energy using first-principles density functional theory calculations. We found that relative fracture energy is directly relevant to the in-plane cleavage direction of CrPS4. It is also found that in CrPS4 the rhomboidal Cr S Cr bonds, which are especially vulnerable in 〈1 1 0〉 direction, connect robust structural motifs to each other. Our study can provide a facile approach to figure out the direction of 2D crystals without complex characterization process, which is valuable for material processing of 2D materials.

The current work introduces a new type of cubic boron nitride with a special sodalite cage structure, named Cubic-B6N6, based on the first principle of density functional theory (DFT). After determining the lattice parameters at the lowest energy state, the phonon spectra and elastic constants were calculated to demonstrate the dynamical and mechanical stability of the structure, respectively. Cubic-B6N6 possesses its bulk modulus of 274.0 GPa, shear modulus of 258.5 GPa, Young’s modulus of 675.8 GPa and Poisson's ratio of 0.09, indicating its large hardness. The electronic band structure indicates that Cubic-B6N6 is an insulator with a wide indirect band gap of 5.97 eV (HSE06). Moreover, XRD and Raman spectra are calculated to predict the experimental characteristics of Cubic-B6N6. Due to the large bulk modulus and large band gap, Cubic-B6N6 may provide potential super-hard and electronic resistance applications.

Designing an efficient membrane for He purification is quite crucial in scientific and industrial applications. Ultrathin membranes with intrinsic pores are highly desirable for gas purification because of their controllable aperture and homogeneous hole distribution. Based on the first-principles density function theory and molecular dynamics simulations, we demonstrate that the compressively strained graphitic carbon nitride (CN) can effectively purify He from Ne and Ar. Under a compressive strain of 6%, the CN monolayer with a suitable pore size presents an easily surmountable barrier for He ( 0.11 eV) but formidable for Ne ( 0.51 eV) and Ar ( 2.45 eV) passing through the membrane, and it exhibits exceptionally high selectivity of 5.17 × 10 6 for He/Ne and 1.89 × 10 39 for He/Ar, as well as excellent He permeance of 1.94 × 10 7 GPU at room temperature, superior to those of porous graphene and C2N membrane. Our results confirm that strain-tuned CN membrane could be potentially utilized for He separating from other noble gases.

In this work, we study the structural phase transitions of gallium phosphide (GaP), witha combination of molecular dynamics simulations (MD) and ab initio calculations based on density functional theory (DFT) with vibrational corrections within the harmonic approximation, for pressures up to 50 GPa. We show that the transition sequence depends on temperature for both cases. For low temperatures, our MD simulations predict a transition sequence zinc blende → β -tin → Immm → NaCl, while DFT predicts the sequence zinc blende → Sc16 → β -tin → Immm. Our DFT calculations with vibrational corrections also indicate that, for high temperatures, the Sc16 phase gradually loses stability and other structures such as Cmcm, β -tin and NaCl become more stable. The kinetic effects are also studied by dynamically changing the pressure in MD simulations up to 200 GPa. We show that, as we increase the pressure, a transition to the β -tin structure should not occur at room temperature due a large energy barrier. For higher temperatures, we show that the first transition is zinc blende → NaCl. The differences in the observed transition sequence at low and high temperatures indicate that the vibrational corrections included in the DFT calculations are fundamental to the description of the stability of higher pressure structures, as also observed on other semiconductors. Moreover, the good agreement between DFT and MD shows the suitability of the classical effective many-body interaction potential, used to describe the different high-pressure phases of GaP.

Interstitial transition metal nitrides and carbides based on simple crystal structures with partial occupation of certain sites by N and C frequently show long-range ordering. In this work the focus is on the important group of superstructures based on a face-centered cubic (fcc)-based arrangement of the metal atoms and octahedral site occupation by N or C. The ordering tendencies are explored for various Ni4N and Ni4C superstructures using density-functional-theory. The ordering tendencies differ strikingly as revealed by the low-energy superstructures encountered for Ni4N and for Ni4C: in the case of Ni4N 2nd-next-neighbor N-N pairs appear favorable whereas in Ni4C 1st-next neighbor C-C pairs are preferred. This can be explained by an interplay between elastic interactions favoring 1st-next neighbor pairs of N and C atoms in an fcc-based arrangement of metal atoms and chemical/Coulomb interactions favoring 2nd-next neighbor (i.e. more distant) pairs. While the elastic interactions are expected to be similar for N and C, the chemical/Coulomb interactions are stronger for N-N as compared to C-C and are thus able to explain the different ordering tendencies.

First-principles density functional theory (DFT) study has been conducted to investigate the probable reaction pathways of carbon formation from ethylene decomposition on Cu(410) surface. The geometric structures and energy information of all the species and relevant C H and C C bond scission reactions have been computed. The results indicate that the proceeding of the C H bond scission reactions is more favorable than C C bond scission reactions. With the aid of the computed results, we find two possible pathways of ethylene decomposition. Ethylene experiences complete dehydrogenation, no carbon monomer will be generated and CC is the final product. The CC species are easy to form but difficult to decompose, which indicates that CC may be another form of carbon deposition. The research may help us make further understanding on the mechanism of carbon formation and be helpful to design of Cu-based catalyst.

We have presented a systematic density functional theory based first principles electronic structure calculations on Ca 1 - x RE x FeAs 2 for various values of x and different rare earth elements (RE = Ce, Pr, Nd, Sm, Gd). Our study on electronic structure reveals that rare earth doping act as heavy electron doping in this compound. In general, due to heavy electron doping caused by rare earth doping hole bands drift below the Fermi level and the associated Fermi surfaces shrink leading to Lifshitz transition for each RE doping, whereas the electron like Fermi surfaces surge. We emphasize on the fact that rare earth doping causes significant modifications to the band widths of Fe derived bands that are known to be responsible for superconductivity. We demonstrate orbital selective band width renormalization and hence orbital selective electron correlation induced by rare earth doping. The variation of band renormalization ratio with increasing rare earth doping reveals a unique connection between orbital selective band renormalization and Lifshitz transition in 112 iron pnictides.

Van der Waals (vdW) heterostructures made by stacking two-dimensional (2D) materials, open up a wide landscape of possibilities in bandgap engineering, due to the distinct properties of the building layers and diverse stacking arrangements possible. In this work, ab initio calculations based on density functional theory were performed to systematically investigate the tuning of electronic properties of GeC/WS2 heterostructures by applying either a vertical external electric field or a biaxial strain. GeC/WS2 heterostructures exhibit a direct band gap and a type-II vdW heterostructure. The bandgap of GeC/WS2 heterostructure can be dramatically modulated by applying a vertical external electric field or a biaxial strain, and semiconductor-metal transitions are observed under high electric field and large strain. The change of band gap is due to the shift of band edges of the heterostructure due to the static electric field induced potential energy and strain induced deformation potential. Differences in the pattern of change style can be assigned to changes of the bands assigned to CBM and VBM. It is interesting to note that the shift of band edge by electric field is screened by a factor of 3.54 as compared to the electrostatic calculation. The deformation potential of the CBM consisted of W 5d orbital is −1.58 eV, and that of the VBM consisted of C 2p orbital is −0.01 eV. In addition, the optical properties of the heterostructure exhibit a significant enhancement compared to the constituent layers, which indicates improved light absorption. The above results suggest that GeC/WS2 vdW heterostructures can find applications as opto-electronic devices and efficient solar energy harvesting materials.

In this article, we reviewed the progress of theoretical and computational studies for confined catalysis in one-dimensional and two-dimensional systems. It was found that the chemical potential of reaction species was usually enhanced in confined space, validated in heterogeneous catalysis and electrocatalysis. Tunable chemical potential of reaction species in confined nanoscale reactors is recognized as a feasible avenue to establish exceptional activity and selectivity of catalytic reactions. A key concept, namely, “confinement energy”, has been defined based on our density functional theory calculations, which is able to unify a number of experimental and computational results. In addition, the concept of “confinement energy” has been able to successfully predict new experimental phenomena.

Electronic excitations and their dynamics are oftentimes at the foundation of how we use and probe materials. While recent experimental advances allow us to do so with unprecedented accuracy and time resolution, their interpretation relies on solid theoretical understanding. This can be provided by cutting-edge, first-principles theoretical-spectroscopy based on many-body perturbation theory (MBPT) and time-dependent density functional theory (TDDFT). In this work we review some of our recent results as successful examples for how electronic-structure methods lead to interesting insight into electronic excitations and deep understanding of modern materials. In many cases these techniques are accurate and even predictive, yet they rely on approximations to be computationally feasible. We illustrate the need for further theoretical understanding, using dielectric screening as an example in MBPT and faster, more accurate numerical integrators as a challenge for real-time TDDFT. Finally, we describe how incorporating online databases into computational materials research on excited electronic states can side-step the problem of high computational cost to facilitate materials design.

The atomic structure of single layer transition metal-dichalcogenides (TMDs) like two-dimensional Vanadium dichalcogenides (VS2 and VSe2) with octahedral (1T) phase has attracted remarkable interest due to its excellent physical and chemical properties. In this work, we conduct density functional theory (DFT) simulations to study the applicability of 1T VS2 and VSe2 as anode materials for Al-, Mg-, Ca-, Na- or Li-ion batteries. To explore the application of 1T VS2 and VSe2 in rechargeable batteries, charge transfer efficiency, adsorption energy, open circuit voltage profile, stability, electronic conductivity and diffusion energy barrier were evaluated. The results revealed that, both 1T VS2 and VSe2 show highly promising performances as anode material for Li-, Ca- or Na-ions batteries, whereas the Al and Mg atoms are not favourable for the adsorption. Consequently, our first-principles analysis reveals that 1T VS2 and VSe2 have considerable charge capacities of 466 mAh/g and 257 mAh/g for Li-, Ca- or Na-ion storages, which is very promising for stretchable rechargeable batteries.

We have studied the pressure-induced structural, magnetic and electronic properties of AeFe2As2 (Ae = Ba, Sr, Ca) compounds in the framework of density functional theory within the GGA-PBE method. The effects of chemical pressure generated by Sr and Ca substitutions in BaFe2As2 have been investigated. We have found a magnetic transition at the same primitive unit cell volume, around 81 A3 for the (Ba ⧹ Ca)Fe2As2 compounds, which predicts a magnetic transition pressure of 12 GPa for SrFe2As2. The structural parameters of FeAs4 tetrahedra are obtained after ionic relaxation and compared with the existing experimental results. The change of these internal parameters is ascribed to pressure-induced charge transfer, i.e., as the As-Ae bonds absorb electrons, the Fe-As bonds are depleted, and thus the number of holes in the Fe-As layers increases. The changes of the density of states, the band structure and the hole- and electron- pockets of Fermi surface by pressure have been discussed.

Iron based industrial steels typically contain a large number of alloying elements, even so-called low alloyed steels. Under irradiation, these alloying elements form clusters that have a detrimental impact of the mechanical properties of the steel. The stability and formation mechanisms of such clusters are presently not fully understood. Therefore, in this work, we study the thermal stability and formation kinetics of small solute clusters in the bcc Fe matrix. We use density functional theory (DFT) to characterize the binding energy of vacancy/solute clusters containing Cr, Mn, Ni, Cu, Si and P, thereby exploring >700 different configurations. The constructed DFT data base is used to fit a cluster expansion (CE) for the vacancy-FeCrMnNiCuSiP system. In turn, the obtained CE is applied in atomistic kinetic Monte Carlo simulations to study the effect of Mn, Ni, Cr, Si and P on the precipitation formation in the FeCu alloy. We conclude that the addition of Mn and Ni delay the precipitation kinetics by an order of magnitude. The additional alloying with traces of P/Si further delays the kinetics by an additional order of magnitude. We found that Si plays an essential role in the formation of spatially mixed MnNiCuSi cluster.

Ti-alloys have many qualities making them ideal for use in aerospace applications, medical implants and chemical industries such as high strength to weight ratio, good high temperature strength and chemical stability. One downside to Ti-alloys is, however, that they are considered difficult to machine. Several investigations have been made in order to understand the wear mechanisms present in machining of Ti-alloys and the most common understanding is a combination of attrition and dissolution-diffusion. Observations by Odelros et al. [1] have shown that there exists a small layer of pure bcc W on top of the outermost WC grains after turning of Ti-6Al-4V. In order for such a layer to form C has to diffuse away from the WC leaving behind only W. In this work Density Functional Theory (DFT) is used together with Harmonic Transition State Theory (HTST) to investigate the pre-factors and barriers for C diffusion into and within two different WC/W interfaces, [0 0 0 1]/[1 1 1] and [1 0 1 ¯ 0]/[1 0 0]. The diffusion into the interfaces show that the barrier for the [0 0 0 1]/[1 1 1] interface is more than twice as high as the barrier for the [1 0 1 ¯ 0]/[1 0 0] interface. Diffusion within the interfaces show, on average, slightly higher barriers for the [0 0 0 1]/[1 1 1] interface.

CuMn2O4 and NiMn2O4 spinels supported on TiO2 were prepared by the impregnation method, and their efficiencies of selective catalytic reduction (SCR) of NOx with NH3 in the simulated flue gas were researched. The two kinds of catalysts showed high NH3-SCR activity for NOx removal over a range of temperature (353–453 K). According to the in situ diffuse reflectance infrared transform spectroscopy (in situ DRIFTS) test, a larger quantity of adsorbed NH3 was detected for CuMn2O4 when NH3 was introduced, and NiMn2O4 provided a larger adsorption amount of NO when NO was introduced. Meanwhile, the corresponding slab models were constructed by density functional theory (DFT) calculations to investigate the adsorption of NH3 and NO molecules. The DFT calculations of NO adsorption were in agreement with the results that were observed by the experiments.

Density functional theory calculations of 〈a〉-type screw dislocation core structures in hcp titanium under a series of applied stresses with zero critical resolved shear stress were performed to investigate the origin of non-Schmid law slip behavior in α-titanium alloys. It was found that compression applied on the [ 1 1 ¯ 0 0 ] direction perpendicular to the Burgers vector leads to more compact pyramidally-oriented cores, while tension applied on the same axis causes an extension of these cores along the prismatic plane. Nudged elastic band calculations of the barrier to slip of the dislocations on the prismatic plane show that extension along this plane leads to reduced slip barriers. This suggests that non-Schmid stresses alter the critical resolved shear stress by inducing changes in the equilibrium core structure.

Niobium (Nb) as a refractory metal has appealing mechanical properties due to high ductility at room temperature. Density functional theory (DFT) calculations confirmed its intrinsic ductility by showing its perfect lattice generates an elastic shear deformation preceding its cleavage fracture under 〈 1 0 0 〉 ideal tensile deformation. Based on DFT calculations of simple atomic configurations, we applied an evolution strategy method to build a modified embedded-atom method (MEAM) spline-interpolation potential that can reproduce the intrinsic ductility of Nb during ideal tensile deformation. Further examinations show that simulations based on this MEAM potential can exhibit other essential characteristics of deformation behavior, such as ideal shear strengths, generalized stacking fault energies, twin boundary energies, phonon spectrum, etc., that are consistent with DFT calculations. This potential also outputs the structures, energetic stability and glide mechanism of 1 2 〈 1 1 1 〉 screw dislocations, such as dislocation core structures, core energies, migration barriers and dislocation core trajectories, consistent with DFT calculations. Therefore, this MEAM potential is suitable to study the mechanical behaviors of Nb, especially those under extreme loading conditions and considering non-Schmid effects.