Ab Initio Density Functional Theory Research Articles

Evaluation of Yttrium Hydride (δ-YH2-x ) Thermal Neutron Scattering Laws and Thermophysical Properties

Yttrium hydride is being considered as a moderator material for microreactor concepts because of its excellent hydrogen retainment capacity at high temperatures. These types of reactors, operating at thermal to epithermal neutron energies, require accurate thermal scattering laws (TSLs) for yttrium hydride to predict and optimize moderator performance. Currently, TSL evaluations exist only for stoichiometric YH2. To perform high-certainty neutronics calculations and to improve the criticality safety of yttrium hydride–moderated reactors, evaluations of substoichiometric yttrium dihydride TSLs are necessary. Ab initio density functional theory (DFT) was used to generate the phonon density of states for yttrium and hydrogen under harmonic approximation in yttrium hydride (). To obtain substoichiometric yttrium dihydride, vacancies in the YH2 crystal were created using special quasi-random structures (SQS). Using NJOY2016, the TSLs for yttrium hydride were constructed from the DFT results as a function of stoichiometry and temperature. Our TSLs for the stoichiometric composition YH2 were in excellent agreement with the ENDF/B-VIII.0 evaluations. As such, this study extends the yttrium hydride TSLs for compositions between YH1.31 to YH1.91 with the interval of H/Y ≈ 0.1 for use in the MCNP code. The substoichiometric yttrium hydride scattering cross sections deviated by as much as 30% (elastic) and 60% (inelastic) when compared to the YH2 TSLs, underlining the necessity to have the TSLs presented here available, e.g., for safety-related reactor calculations. For the validation of the underlying DFT results of our model, quasi-harmonic approximation was used to compute the thermal lattice strain and constant pressure heat capacity for YH2. Neutron diffraction experiments were also carried out to characterize thermophysical properties that were adopted for stoichiometric and substoichiometric model validation. Additional properties such as heat capacity cv, and thermal displacement parameters were also computed for yttrium hydride () and compared to experimental results. Neutron diffraction validation of the YH2-x material properties and ENDF/B-VIII.0 verification of YH2 TSLs provide a very strong basis on the accuracy of the extended yttrium hydride TSL evaluations at thermal energies.

Theoretically modelling graphene-like carbon matryoshka with strong stability and particular three-center two-electron π bonds.

Carbon materials based on different hybridization of carbon atoms have drawn great attention because of their unique configurations and physical and chemical properties. Here, a previously unknown 2D carbon allotrope named L-2Gy, graphene-like carbon matryoshka graphynes (Gy) with two alkynyls (C[triple bond, length as m-dash]C) inserted into the three-fold carbon atoms of graphene, has been constructed with considerable thermal, dynamical, and mechanical stability by using ab initio density functional theory. With the increasing number of alkynyls between the three-fold carbon atoms of graphene, the stability of Gy will seriously decrease. L-2Gy has a fascinating chemical bond environment consisting of sp- and sp2-hybridized carbon atoms, and delocalized π electrons derived from the 27 three-center two-electron π bonds. This particular electronic structure plays a vital role in chemically stabilizing L-2Gy. The electronic band structure reveals the semi-metallic features of L-2Gy mainly contributed by the px/z orbitals of carbon atoms. Furthermore, compared with the acknowledged catalysts for the hydrogen evolution reaction (HER), L-2Gy, as a 2D carbon allotrope, shows excellent catalytic activity for the HER.

Rare-earth-free ferrimagnetic Mn4N sub-20 nm thin films as potential high-temperature spintronic material

Ferrimagnetic alloy thin films that exhibit perpendicular (out-of-plane) magnetic anisotropy (PMA) with low saturation magnetization, such as GdCo and Mn4N, were predicted to be favorable for hosting small Néel skyrmions for room temperature applications. Due to the exponential decay of interfacial Dzyaloshinskii–Moriya interaction and the limited range of spin–orbit torques, which can be used to drive skyrmion motion, the thickness of the ferrimagnetic layer has to be small, preferably under 20 nm. While there are examples of sub-20 nm, rare earth-transition metal (RE-TM), ferrimagnetic thin films fabricated by sputter deposition, to date, rare-earth-free sub-20 nm Mn4N films with PMA have only been reported to be achieved by molecular beam epitaxy, which is not suitable for massive production. Here, we report the epitaxial growth of sub-20 nm Mn4N films with PMA at 400 °C–450 °C substrate temperatures on MgO substrates by reactive sputtering. The Mn4N films were achieved by reducing the surface roughness of MgO substrate through a high-temperature vacuum annealing process. The optimal films showed low saturation magnetization (Ms = 43 emu/cc), low magnetic anisotropy energy (0.7 Merg/cc), and a remanent magnetization to saturation magnetization ratio (Mr/Ms) near 1 at room temperature. Preliminary ab initio density functional theory calculations have confirmed the ferrimagnetic ground state of Mn4N grown on MgO. The magnetic properties, along with the high thermal stability of Mn4N thin films in comparison with RE-TM thin films, provide the platform for future studies of practical skyrmion-based spintronic materials.

Discrete Kernel Functions for fcc Crystals Within Eringen’s Nonlocal Theory of Elasticity

The dilemma with the deficiencies of the nonlocal kernel functions as the building blocks of the Eringen’s nonlocal theory has been of concern. The aim of the current work is to provide a remedy for the calculation of the components of the nonlocal moduli tensor pertinent to face center cubic (fcc) crystals accounting for their true symmetry group. To this end, three new distinct nonlocal kernel functions which are the discrete atomistic Green’s functions in the stress space are obtained through the nonlocal dispersion relations associated with the longitudinal and shear waves in fcc crystals combined with the corresponding ones calculated via ab initio based on density functional perturbation theory (DFPT). In other words, based on nonlocal continuum and quantum mechanical considerations different kernel functions pertinent to each component of the elastic moduli tensor are calculated for the first time. Moreover, the nonzero components of the classical elastic moduli as well as the equilibrium lattice parameter associated with Al, Cu, Ni, Pd, and Ag fcc crystals have been computed using ab initio density functional theory (DFT) and compared with the available experimental data. In order to show the importance of the anisotropy of the kernel functions, the nonlocal stress field of a screw dislocation inside an infinitely extended fcc medium has been calculated; the solutions for two cases where the dislocation line is along the $\left [ 0\ 0\ 1 \right ]$ and $\left [ 1\ 0\ 1 \right ]$ directions are compared. Within this theory the unrealistic singular behavior of the classical stresses at the dislocation core is eliminated.

Flexoelectricity in atomic monolayers from first principles.

We study the flexoelectric effect in fifty-four select atomic monolayers using ab initio Density Functional Theory (DFT). Specifically, considering representative materials from each of the Group III monochalcogenides, transition metal dichalcogenides (TMDs), Groups IV, III-V, and V monolayers, Group IV dichalcogenides, Group IV monochalcogenides, transition metal trichalcogenides (TMTs), and Group V chalcogenides, we perform symmetry-adapted DFT simulations to calculate transversal flexoelectric coefficients along the principal directions at practically relevant bending curvatures. We find that the materials demonstrate linear behavior and have similar coefficients along both principal directions, with values for TMTs being up to a factor of five larger than those of graphene. In addition, we find electronic origins for the flexoelectric effect, which increases with monolayer thickness, elastic modulus along the bending direction, and sum of polarizability of constituent atoms.

Theoretical study of the band structure of 2H-SiC and 4H-SiC of silicon carbide polytypes

We have studied the electronic band properties of 2H-SiC and 4H-SiC silicon carbide polytypes. The structures of the electronic bands and density of state (DOS) using ab initio Density Functional Theory (DFT) were calculated for the first Brillouin zone both in the generalized gradient approximation and taking into account quasiparticle effects according to the GW scheme. The calculated bandgaps obtained using the GW approximation Eg2H-SiC = 3.17 eV and Eg4H-SiC = 3.26 eV agree well with experimental values. The shape and values of total DOS are within agreement with calculations performed by other authors. The calculated total energy values for 2H-SiC and 4H-SiC were close, but they satisfy the condition E2H > E4H, which implies that the 4H-SiC structure is more stable than 2H-SiC. Our calculations of the band structure and DOS of 2H-SiC and 4H-SiC by the DFT method showed that the application of the GW approximation is an optimum approach to the study of the electronic structure of 2H-SiC and 4H-SiC polytypes.

Theoretical investigation of the electronic structure, elastic, dynamic properties of intermetallic compound NiBe under pressure

From the beryllide group, NiBe is a hybrid material of CsCl (B2) type structure and presents a large range of stoichiometry. In this paper, we have studied the structural, electronic, elastic, and dynamic properties of NiBe intermetallic compound under pressure by performing ab initio density functional theory using the Cambridge Serial Total Energy Package (CASTEP). In B2 structure, we have obtained the lattice constant ( $${A}_{\mathrm {0}}$$ ), equilibrium volume ( $${V}_{\mathrm {0}}$$ ), bulk modulus ( $${B}_{\mathrm {0}}$$ ) and its pressure derivative ( $${B}_{0}$$ ’) and elastic constants. We have quantitatively analyzed the band structure and the corresponding density of states (DOS). We have also estimated the mechanical stability and brittle/ductile behaviors of this compound based on the computed elastic constants. It is found that this structure is stable between 0 and 50 GPa pressure for NiBe compound. Finally, the changing degree of anisotropy with pressure was analyzed in three and two dimensions.

Study of some electronic and spectroscopic properties of ZnO nanostructers by density functional theory

The electronic and spectroscopic properties of ZnO nanostructures were evaluated by ab-initio density functional theory and generalized gradient approximation calculations combined with many-body, all calculations using Gaussian 09 program. These electronic properties including energy gap, density of state and bond length were calculated, also the spectroscopic properties like infrared and Raman scattering, force constant and reduced mass were calculated and compared with longitudinal optical mode of vibration. The geometrical structure of ZnO molecules and wurtzoids nanostructures were studied with the help of using Gauss view 05 program. The energy gaps of cyclohexane, Wurtzoid, wurtzoid2C and tri-wurtzoid are (4.48, 3.35, 3.4 and 3.096 eV) respectively, these values agreement with experimental values for ZnO wurtzite bulk (3.3 eV). As well as, the Spectroscopic study of IR and Raman scattering, force constant, and reduced mass as a function of frequencies showed an agreement with the experimental results.

Diameter Dependent Structural and Electronic Properties of TiC Nanotubes

Here, we report a systematic and comparative analysis of cylindrical single walled Titanium Carbide (TiC) Nanotubes with different structural configuration (zigzag and armchair), by Ab-initio density functional theory (DFT) using generalised gradient approximation (GGA) with revised Perdew Burke Ernzerhoff (rPBE) type parameterization. The variation of structural stability via binding energy (BE) and electronic properties via band structure and density of states (DOS) are analysed with increasing diameter of Nanotubes. Interestingly, the result obtained from our calculation shows TiC Nanotubes is semiconducting, while its bulk counterpart is metallic in nature. In results, the Computed band gap and binding energy, both decreases with increase in diameter of all types of NT and saturated after some values. These studies explore several opportunities for further studies that are touched upon.

Study geometrical, electronic and spectroscopic properties of BeO wurtzoids via DFT

The electronic and spectroscopic properties of BeO wurtzoids structure were investigated by using ab-initio density functional theory and generalized gradient approximation calculations combined with many-body. All calculations were carried out using Gaussian 09 program. Electronic properties including energy gap, density of state and bond length, as well as Spectroscopic properties of infrared and Raman scattering, force constant, reduce mass and longitudinal optical patterns. The geometrical structure of BeO molecules and wurtzoids nanostructure were studied with the help using Gauss view 05 program. The energy gap of Wurtzoid, wurtzoid2c and triwurtzoid was found to be (6.813, 7.13499 and 7.02669 eV) respectively. Those values agreed with theoretical values for BeO wurtzite (8.57 eV), and compare with the experimental value of BeO bulk (10.6 eV). The Spectroscopic study of IR and Raman scattering, force constant, and reduced mass and longitudinal optical modes as a function of frequencies are agreed with the experimental results.

Molecular and nano structures of chiral PEDOT derivatives influence the enantiorecognition of biomolecules. In silico analysis of chiral recognition.

In this study we investigated the synergistic effects of the chirality (molecular structure) and surface morphology (nanostructure) of a newly designed sensing platform for the stereoselective recognition of biomolecules. We synthesized 3,4-ethylenedioxythiophene monomers presenting an OH functional group on the side chain (EDOT-OH) with either R or S chirality and then electropolymerized them in a template-free manner to engineer poly(EDOT-OH) nanotubes and smooth films with R or S chirality. We used a quartz crystal microbalance (QCM) to examine the differential binding of fetal bovine serum, RGD peptide, insulin, and (R)- and (S)-mandelic acid (MA) on these chiral polymeric platforms. All of these biomolecules bound stereoselectively and with greater affinity toward the nanotubes than to the smooth films. The sensitive chiral recognition of (S)- and (R)-MA on the (R)-poly(EDOT-OH) nanotube surface occurred with the highest chiral discrepancy ratio of 1.80. In vitro experiments revealed a greater degree of protein deposition from MCF-7 cells on the chiral nanotube surfaces. We employed ab initio molecular dynamics simulations and density functional theory calculations to investigate the mechanism underlying the sensitive chiral recognition between the chiral sensing platforms and the chiral analyte molecules.

Controlling the aqueous growth of urea crystals with different growth inhibitors: a molecular-scale study.

Molecular scale understanding of the mechanism of solution-mediated nucleation and the growth of crystalline materials in the presence of growth inhibitors together with the process parameters continues to attract the interest of the scientific community though much headway has been made in recent years. Growth inhibitors can be added to solution of a crystallizing parent molecule to alter the rate of growth of different crystal faces, size and shape of the crystalline materials. In this work, we investigated the effects of a number of shape-controlling inhibitors, such as acetone, biuret and biurea, on the growth kinetics of the various faces of aqueous-grown urea crystals as a means to predictably control the crystal growth morphology. We combined the adsorption energy landscape of various auxiliaries with the kinetics of the molecular growth processes to develop an analytical model to compute the rate of growth as a function of supersaturation and the additive concentration. The model relates the kinetic and thermodynamic aspects of the adsorption of the solute, solvent and additive to provide a quantitative description of the crystal growth. Ab initio periodic dispersion-corrected density functional theory using the hybrid exchange–correlation functional was employed to determine the interfacial structure of the adsorption of various auxiliaries at crystalline surfaces. The calculated adsorption energies of different auxiliaries were employed to examine the role played by these auxiliaries during the aqueous crystallization of urea crystals containing small amounts of additives. Our results showed that the growth of (110), (111) and (1̄1̄1̄) faces were nearly unaltered by the addition of moderate amounts of acetone as it has lower adsorption energies with the surfaces of these faces. Nevertheless, the presence of acetone in the solution reasonably impeded the growth of the (001) face. The addition of biuret or biurea in the solution led to a higher adsorption energy at (001) and (111) faces. Consequently, the low concentration of these additives severely obstructed the growth of (001) and (111) faces as most of the adsorption sites were occupied by these additives. On the other hand, these additives were weakly adsorbed at the (110) face and, hence, the growth of the (110) face largely remained unaltered. Moreover, unlike biuret, biurea considerably inhibited the growth of the (1̄1̄1̄) face. Our results are in agreement with the experimental and computational results reported in the literature.

A new 2D auxetic CN2 nanostructure with high energy density and mechanical strength.

The existence of a new two dimensional CN2 structure was predicted using ab initio molecular dynamics (AIMD) and density-functional theory calculations. It consists of tetragonal and hexagonal rings with C-N and N-N bonds arranged in a buckling plane, isostructural to the tetrahex-carbon allotrope. It is thermodynamically and kinetically stable suggested by its phonon spectrum and AIMD. This nanosheet has a high concentration of N and contains N-N single bonds with an energy density of 6.3 kJ g-1, indicating its potential applications as a high energy density material. It possesses exotic mechanical properties with a negative Poisson's ratio and an anisotropic Young's modulus. The modulus in the zigzag direction is predicted to be 340 N m-1, stiffer than those of h-BN and penta-CN2 sheets and comparable to that of graphene. Its ideal strength of 28.8 N m-1 outperforms that of penta-graphene. The material maintains phonon stability upon the application of uniaxial strain up to 10% (13%) in the zigzag (armchair) direction or biaxial strain up to 5%. It possesses a wide indirect HSE band gap of 4.57 eV, which is tunable between 3.37-4.57 eV through strain. Double-layered structures are also explored. Such unique properties may facilitate its potential applications as a high energy density material and in nanomechanics and electronics.

Ab-Initio Investigation of Planar Defects in <i>Immm</i>-Ni <sub>2</sub>(Cr,Mo,W) Strengthened HAYNES 244 Alloy

The Immm-Ni2(Cr,Mo,W) body centered orthorhombic (BCO) intermetallic phase, denoted as γ´´´, is the primary strengthening phase in commercial Ni-base HAYNES® 242® and 244® alloys. Due to the relatively low symmetry and six crystallographic variants precipitating in the face-centered cubic solid solution γ matrix, the deformation mechanisms are expected to be complex, and a myriad of planar defects are predicted to be observed on the close-packed {013} and {110} planes, which are nearly co-planar to the matrix {111} plane. We find that these defects include those analogous to the γ´-Ni3Al phase: superlattice intrinsic stacking faults, antiphase boundaries, and complex stacking faults. We determined these planar defect energies and generalized stacking fault energy surfaces utilizing ab-initio density functional theory calculations. The γ´´´ {013} and {110} planes exhibited comparable planar defect energies but showed drastically different dislocation shear pathways due to the lower symmetry of the orthorhombic phase. We observed that the addition of W increased the fault energies significantly, which could correspond to the observed increase in yield strength of 244 alloy over that of 242 alloy.

Identifying alternative ferroelectric materials beyond Hf(Zr)O2

A database-driven approach combined with ab initio density functional theory (DFT) simulations is used to identify and simulate alternative ferroelectric materials beyond Hf(Zr)O2. The database-driven screening method identifies a class of wurtzite ferroelectric materials. DFT simulations of wurtzite magnesium chalcogenides, including MgS, MgSe, and MgTe, show their potential to achieve improved ferroelectric (FE) stability, simple atomistic unit cell structure, and large FE polarization. Strain engineering can effectively modulate the FE switching barrier height for facilitating FE switching. The effect of the piezoelectric property on the FE switching barrier heights is also examined.

Benchmarking Quantum Mechanical Methods for the Description of Charge-Transfer States in π-Stacked Nucleobases.

Charge-transfer (CT) states are of special interest in photochemical research because they can facilitate chemical reactions through the rearrangement of electrons and subsequently chemical bonds in a molecular system. Of particular importance to this research is the transfer of electrons between π-stacked nucleobases in DNA because they play an important role in its photophysics and photochemistry. Computational methods are paramount for the study of CT states because of the current inability of experimental methods to easily detect such states. However, many ab-initio wavefunction-based and density functional theory (DFT) methods fail to accurately describe these CT states. Here, we benchmark how 40 different quantum mechanical methods describe the excited states of a guanine-thymine π-stacked nucleobase dimer system, both in 5'-TG-3' and 5'-GT-3' conformations. We find that the distance between the nucleobases plays a major role in the energy of the CT state and in the difference of the dipole moments between the CT and ground state. There is a larger range of values (and errors) for the energies of CT states compared to those of states localized on one nucleobase. Wavefunction-based methods have similar errors for the CT and localized valence states, while DFT methods are very sensitive to the amount of Hartree-Fock exchange. Long-range-corrected functionals with a careful balance of the Hartree-Fock exchange included can predict very accurate CT states and a balanced description with the localized states.

Interplay between CO Disproportionation and Oxidation: On the Origin of the CO Reaction Onset on Atomic Layer Deposition-Grown Pt/ZrO2 Model Catalysts.

Pt/ZrO2 model catalysts were prepared by atomic layer deposition (ALD) and examined at mbar pressure by operando sum frequency generation (SFG) spectroscopy and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) combined with differentially pumped mass spectrometry (MS). ALD enables creating model systems ranging from Pt nanoparticles to bulk-like thin films. Polarization-dependent SFG of CO adsorption reveals both the adsorption configuration and the Pt particle morphology. By combining experimental data with ab initio density functional theory (DFT) calculations, we show that the CO reaction onset is determined by a delicate balance between CO disproportionation (Boudouard reaction) and oxidation. CO disproportionation occurs on low-coordinated Pt sites, but only at high CO coverages and when the remaining C atom is stabilized by a favorable coordination. Thus, under the current conditions, initial CO oxidation is found to be strongly influenced by the removal of carbon deposits formed through disproportionation mechanisms rather than being determined by the CO and oxygen inherent activity. Accordingly, at variance with the general expectation, rough Pt nanoparticles are seemingly less active than smoother Pt films. The applied approach enables bridging both the “materials and pressure gaps”.

Theoretical analysis of the reactivity of chloroquine and hydroxychloroquine

Chloroquine (CQ) and Hydroxychloroquine (HCQ) have a low safety margin and its toxic effects are closely related to the ingested dose. Both drugs were tested for reactivity under different conditions but still need to be understood. A thermodynamic and kinetic study with control the electronic properties also show the reaction of the molecule. In this study, theoretical calculations have been performed using Density Functional Theory (DFT) and Hartree-Fock (HF) to find the band gap energy and determine a suitable basis set. A computation based on B3LYP level was accomplished to obtain the geometrical structures for both CQ and HCQ molecules. Based on the B3LYP/6-31G(d,p) basis set, DFT measurements of frontier molecular orbitals and molecular electrostatic potentials have been implemented for both CQ and HCQ. The atomic charge distribution of nitrogen and oxygen is calculated for CQ and HCQ using DFT and HF on a basis set 6-31G**. Ab initio DFT with HF at 6-31G** basis set is performed for thermodynamic analysis for both CQ and HCQ structures.

Observing the Fluctuation Dynamics of Dative Bonds Using Two-Dimensional Electronic Spectroscopy.

We perform two-dimensional electronic spectroscopy on chlorophyll (Chl) a and b molecules in aprotic solvents of different Lewis basicity. By analyzing the ultrafast spectral diffusion dynamics of the Qy transition, we show that a certain timescale of the spectral diffusion dynamics is affected by the solvents' Lewis basicity. Control experiments with Chlorin-e6-a Chl molecule analog-and ab initio time-dependent density functional theory calculations confirm that we are directly probing the fluctuation dynamics of the dative bond between the solvent's lone pair and the Mg2+ center in Chls that is responsible for the Lewis basicity. The observation is indicative of dative bond length and angular fluctuations with timescales ranging between ∼30 and 150 ps and the dative bond-strength-dependent perturbation on the Qy transition frequency of Chls.

Computational determination the reactivity of salbutamol and propranolol drugs

Gaussian software programs 09 was utilized to find the reactivity of salbutamol (SAL) and propranolol (PRO). Density Functional Theory (DFT) and Hartree-Fock (HF) were used to determine the energy band gaps. B3LYP/6-31++G(d,p) lower energy level was chosen as the base set. Geometrical structures with frontier molecular orbitals estimation for both the SAL and PRO. Atomic charge distribution and molecular electrostatic potential evaluation were performed for both drugs. For thermodynamic analysis Ab-initio DFT with HF at 6-31++G base sets were accomplished. The results showed that the PRO is more reactive than SAL.