Self-consistent Density Functional Theory Research Articles

Theoretical study of triaxial shapes of neutron-rich Mo and Ru nuclei

Background: Recently, transition quadrupole moments in rotational bands of even-mass neutron-rich isotopes of molybdenum and ruthenium nuclei have been measured. The new data have provided a challenge for theoretical descriptions invoking stable triaxial deformations. Purpose: To understand experimental data on rotational bands in the neutron-rich Mo-Ru region, we carried out theoretical analysis of moments of inertia, shapes, and transition quadrupole moments of neutron-rich even-even nuclei around $^{110}$Ru using self-consistent mean-field and shell model techniques. Methods: To describe yrast structures in Mo and Ru isotopes, we use nuclear Density Functional Theory (DFT) with the optimized energy density functional UNEDF0. We also apply Triaxial Projected Shell Model (TPSM) to describe yrast and positive-parity, near-yrast band structures. Results: Our self-consistent DFT calculations predict triaxial ground-state deformations in $^{106,108}$Mo and $^{108.110,112}$Ru and reproduce the observed low-frequency behavior of moments of inertia. As the rotational frequency increases, a negative-$\gamma$ structure, associated with the aligned $\nu(h_{11/2})^2$ pair, becomes energetically favored. The computed transition quadrupole moments vary with angular momentum, which reflects deformation changes with rotation; those variations are consistent with experiment. The TPSM calculations explain the observed band structures assuming stable triaxial shapes. Conclusions: The structure of neutron-rich even-even nuclei around $^{110}$Ru is consistent with triaxial shape deformations. Our DFT and TPSM frameworks provide a consistent and complementary description of experimental data.

The Oxidation of Methanol on PtRu(111): A Periodic Density Functional Theory Investigation

Self-consistent periodic density functional theory (PW91-GGA) calculations are employed to study the oxidation of methanol on PtRu(111). Geometries and energies for all the intermediates involved are analyzed, and the oxidation network is mapped out to illustrate the reaction mechanism. On PtRu(111), the Ru atoms with less electronegativity are more favorable to binding the adsorbates than the Pt atoms. Alloying Pt with Ru weakens the bond of CO to Pt, but strengthens the bond of CO to Ru. All possible pathways through initial C–H, O–H, and C–O bond scissions are considered. The initial O–H bond scission is found to be the most favorable and bears an energy barrier comparable to that for methanol desorption. The further oxidation occurs preferentially via the non-CO path from species CHO. The most possible reaction pathway of methanol on PtRu(111) is CH3OH → CH3O → CH2O → CHO → CHOOH → COOH → CO2. Furthermore, the activation of H2O on PtRu(111) is more favorable than that on the pure Pt(111) surface. The ...

Can Multiconfigurational Self-Consistent Field Theory and Density Functional Theory Correctly Predict the Ground State of Metal-Metal-Bonded Complexes?

The electronic structure of a diiron (FeFe) complex with strong metal-metal interaction and those of analogous complexes (CoCo, CoMn, CoFe, and FeMn) with much weaker metal-metal bonding are investigated with wave function-based methods and density functional theory. The delocalization and bonding between the metal centers in the diiron complex is only fully captured after inclusion of the complete set of 3d and 4d orbitals in the active space, a situation best suited for restricted active space (RAS) approaches. Truncation of the included set of 4d orbitals results in inappropriate localization of some 3d orbitals, incorrect description of the ground spin state as well as wrong spin state energetics, as compared to experiment. Using density functional theory, some local functionals are able to predict the correct ground spin states, and describe the chemical bonding and structural properties of all the metal-metal complexes considered in this work. In contrast, the introduction of some exact exchange results in increased localization of 3d orbitals and wrong spin state energetics, a situation that is particularly troublesome for the diiron complex.

Molecular dynamics of photodissociation: towards more complex systems

SynopsisWe present a combined experimental and theoretical study of the photodissociation of thiophene mole-cule using energy-resolved electron-ion-ion coincidence technique and self-consistent charge density functional tight-binding theory combined with a statistical approach. The observed complex molecular dynamics with many internal-energy-dependent fragmentation pathways is successfully described by the theoretical simulations.

An ab-initio study of adsorption of gaseous molecules on doped graphene structures

In this work, electronic properties of bare and doped graphene layers in the presence of N, B and a type of defective impurities are investigated under adsorption of CO, NO, NH3 and NO2 molecules on it's external surface. We use a fully self-consistent density function theory (DFT) based calculations as implemented in SIESTA package. The local-density approximation (LDA) is considered for the exchange-correlation function. Total density of state (TDOS), partial density of state (PDOS) and charge density calculations are also considered to elucidate the difference in the CO, NO, NH3 and NO2 gases detection mechanism of pristine and doped graphene structures. With regard to that the charge transfer is occurring between the graphene sheet and gaseous molecules, the NO2 and NH3 molecules are considered as the recipient and donor of electrons, respectively. We show that the states contributed by the adsorbed CO and NO molecules are quite localized near the center of original band gap and they suggest that the charge transport in such systems cannot be enhanced considerably, while NO2 and NH3 molecules adsorption acts as the acceptor and donor, respectively. Our results can serve as a base for developments in designing nano-electronic devices.

Extended Lagrangian Density Functional Tight-Binding Molecular Dynamics for Molecules and Solids

A computationally fast quantum mechanical molecular dynamics scheme using an extended Lagrangian density functional tight-binding formulation has been developed and implemented in the DFTB+ electronic structure program package for simulations of solids and molecular systems. The scheme combines the computational speed of self-consistent density functional tight-binding theory with the efficiency and long-term accuracy of extended Lagrangian Born-Oppenheimer molecular dynamics. For systems without self-consistent charge instabilities, only a single diagonalization or construction of the single-particle density matrix is required in each time step. The molecular dynamics simulation scheme can be applied to a broad range of problems in materials science, chemistry, and biology.

The effect of particle size on the morphology and thermodynamics of diblock copolymer/tethered-particle membranes.

A combination of self-consistent field theory and density functional theory was used to examine the effect of particle size on the stable, 3-dimensional equilibrium morphologies formed by diblock copolymers with a tethered nanoparticle attached either between the two blocks or at the end of one of the blocks. Particle size was varied between one and four tenths of the radius of gyration of the diblock polymer chain for neutral particles as well as those either favoring or disfavoring segments of the copolymer blocks. Phase diagrams were constructed and analyzed in terms of thermodynamic diagrams to understand the physics associated with the molecular-level self-assembly processes. Typical morphologies were observed, such as lamellar, spheroidal, cylindrical, gyroidal, and perforated lamellar, with the primary concentration region of the tethered particles being influenced heavily by particle size and tethering location, strength of the particle-segment energetic interactions, chain length, and copolymer radius of gyration. The effect of the simulation box size on the observed morphology and system thermodynamics was also investigated, indicating possible effects of confinement upon the system self-assembly processes.

Adsorption and dissociation of H2S on Mo2C(001) surface-A first-principle study

The adsorption and decomposition reaction mechanisms of H2S on Mo2C(001) has been systematically studied using self-consistent periodic density functional theory. Results show that the molecular of H2S is adsorbed either on the Mo top site or bridge site. Mulliken population analysis and density of states for H2S/Mo-terminated Mo2C(001) adsorption system are examined to confirm the adsorption mechanism of H2S with the Mo2C(001) surface, which can involve the donation of charge from the “s lone pair electrons” that are LUMO orbitals into the surface and the back donation of electrons from the surface into the HOMO orbital. The optimal pathway for the dissociation of H2S on the Mo2C(001) surface can be H2Stop→SHfcc+Hfcc→Sfcc+Hfcc+Hfcc. The first step is the rate-determining step because it has the smallest rate constant among the possible reactions pathways.

NO Electrochemical Reduction on Pt Electrocatalysts: A DFT Approach

Electrochemical denitrification is a promising technology for the removal of toxic nitrate and nitrite species from groundwater due to the process’s environmental compatibility, energy efficiency, safety, and product selectivity. The adsorbed NO is generally considered to be a selectivity-determining species during this electrochemical potential dependent reaction. Atomic-scale studies using density functional theory (DFT), in turn, can provide powerful molecular-level information about the elementary mechanisms of reductive NO electrocatalysis. These basic building blocks can further serve as a starting point for future trends-based analysis on different transition metal surfaces. In this work, we employ periodic, self-consistent DFT calculations to clarify the adsorption structures and thermochemistry of NO and its related reaction intermediate species on different Pt single crystal surfaces ((111) and (100)). Kinetics are further determined by calculating the activation barriers for N-O dissociation, protonation, and N-N bond formation. We begin by describing results for the reaction at low NO coverage (ca. 0.11 ML). After describing the thermodynamics (the adsorption) and kinetics (N-O bond breaking and protonation barrier) for the dentirification reaction scheme, a plausible mechanistic reaction pathway is proposed. Water-assisted protonation barriers are generally shown to be lower than barriers for non-electrochemical reactions, such as surface hydrogenation or N-O bond breaking. Consistent with available experimental evidence, from the most probable reaction pathway, we conclude that ammonium ion would be the most favorable product that would evolve. There is no evidence for the formation of hydroxylamine (H2NOH) at low coverage. At higher NO coverages (ca. 0.45 ML), the relative energy states of the intermediates show a similar trend for the energetics when it is compared with the study with lower NO coverage. Finally, N2O formation is examined. We find that trans NO dimer species could be a precursor state for the N­2O formation due to its lower kinetic barrier than barriers that are found for cis-type NO dimers. The simulation demonstrates that N2O could be formed at higher potentials through two protonations of trans NO species, and the reaction is thermodynamically more favorable at higher NO coverage. This barrier could be surmountable at room temperature, and these mechanistic conclusions hold on both the (111) and (100) surfaces. Based on our study, we find that reduction of NO proceeds through the water-assisted protonations which will finally lead to ammonia. The primary pathway for N2O formation occurs through trans-type NO dimer species. The proposed pathways occur with different applied electrochemical potentials and NO coverages.

Pygmy resonances and radiative nucleon captures for stellar nucleosynthesis

The impact of low-energy multipole excitations and pygmy resonances on radiative neutron and proton-capture cross sections in nuclei close to the $\ensuremath{\beta}$-stability line is investigated. For this purpose, a microscopic theoretical approach based on self-consistent density functional theory and quasiparticle-random-phase-approximation formalism extended with multiphonon degrees of freedom is implemented in a statistical reaction model. The advantage of the method is the microscopic nuclear structure input for unified description of low-energy multiphonon excitations and pygmy and giant resonances. This is found to be important for the understanding of the fine structure and dynamics of the nuclear response function at low energies, which strongly influences nuclear reaction rates of astrophysical relevance. Calculations of the radiative capture cross sections of the reactions $^{85}\mathrm{Kr}(n,\ensuremath{\gamma})^{86}\mathrm{Kr}, ^{87}\mathrm{Sr}(n,\ensuremath{\gamma})^{88}\mathrm{Sr}$, and $^{89}\mathrm{Y}(p,\ensuremath{\gamma})^{90}\mathrm{Zr}$ are discussed in comparison with experimental data. For the reactions $^{89}\mathrm{Zr}(n,\ensuremath{\gamma})^{90}\mathrm{Zr}$ and $^{91}\mathrm{Mo}(n,\ensuremath{\gamma})^{92}\mathrm{Mo}$ theoretical predictions of the reaction cross sections are made.

Exchange parameters of strongly correlated materials: Extraction from spin-polarized density functional theory plus dynamical mean-field theory

In this paper we present an accurate numerical scheme for extracting interatomic exchange parameters (${J}_{ij}$) of strongly correlated systems, based on first-principles full-potential electronic structure theory. The electronic structure is modeled with the help of a full-potential linear muffin-tin orbital method. The effects of strong electron correlations are considered within the charge self-consistent density functional theory plus dynamical mean-field theory. The exchange parameters are then extracted using the magnetic force theorem; hence all the calculations are performed within a single computational framework. The method allows us to investigate how the ${J}_{ij}$ parameters are affected by dynamical electron correlations. In addition to describing the formalism and details of the implementation, we also present magnetic properties of a few commonly discussed systems, characterized by different degrees of electron localization. In bcc Fe, treated as a moderately correlated metal, we found a minor renormalization of the ${J}_{ij}$ interactions once the dynamical correlations are introduced. However, generally, if the magnetic coupling has several competing contributions from different orbitals, the redistribution of the spectral weight and changes in the exchange splitting of these states can lead to a dramatic modification of the total interaction parameter. In NiO we found that both static and dynamical mean-field results provide an adequate description of the exchange interactions, which is somewhat surprising given the fact that these two methods result in quite different electronic structures. By employing the Hubbard-I approximation for the treatment of the $4f$ states in hcp Gd we reproduce the experimentally observed multiplet structure. The calculated exchange parameters result in being rather close to the ones obtained by treating the $4f$ electrons as noninteracting core states.

Atomic layer-by-layer deposition of platinum on palladium octahedra for enhanced catalysts toward the oxygen reduction reaction.

We systematically evaluated two different approaches to the syntheses of Pd@PtnL (n = 2-5) core-shell octahedra. We initially prepared the core-shell octahedra using a polyol-based route by titrating a Pt(IV) precursor into the growth solution containing Pd octahedral seeds at 200 °C through the use of a syringe pump. The number of Pt atomic layers could be precisely controlled from two to five by increasing the volume of the precursor solution while fixing the amount of seeds. We then demonstrated the synthesis of Pd@PtnL octahedra using a water-based route at 95 °C through the one-shot injection of a Pt(II) precursor. Due to the large difference in reaction temperature, the Pd@PtnL octahedra obtained via the water-based route showed sharper corners than their counterparts obtained through the polyol-based route. When compared to a commercial Pt/C catalyst based upon 3.2 nm Pt particles, the Pd@PtnL octahedra prepared using both methods showed similar remarkable enhancement in terms of activity (both specific and mass) and durability toward the oxygen reduction reaction. Calculations based upon periodic, self-consistent density functional theory suggested that the enhancement in specific activity for the Pd@PtnL octahedra could be attributed to the destabilization of OH on their PtnL*/Pd(111) surface relative to the {111} and {100} facets exposed on the surface of Pt/C. The destabilization of OH facilitates its hydrogenation, which was found to be the rate-limiting step of the oxygen reduction reaction on all these surfaces.

Effects of electron correlations on transport properties of iron at Earth's core conditions.

Earth's magnetic field has been thought to arise from thermal convection of molten iron alloy in the outer core, but recent density functional theory calculations have suggested that the conductivity of iron is too high to support thermal convection, resulting in the investigation of chemically driven convection. These calculations for resistivity were based on electron-phonon scattering. Here we apply self-consistent density functional theory plus dynamical mean-field theory (DFT+DMFT) to iron and find that at high temperatures electron-electron scattering is comparable to the electron-phonon scattering, bringing theory into agreement with experiments and solving the transport problem in Earth's core. The conventional thermal dynamo picture is safe. We find that electron-electron scattering of d electrons is important at high temperatures in transition metals, in contrast to textbook analyses since Mott, and that 4s electron contributions to transport are negligible, in contrast to numerous models used for over fifty years. The DFT+DMFT method should be applicable to other high-temperature systems where electron correlations are important.

Excited state proton transfer dynamics of thioacetamide in S2(ππ*) state: resonance Raman spectroscopic and quantum mechanical calculations study.

The photophysics and photochemistry of thioacetamide (CH3CSNH2) after excitation to the S2 electronic state were investigated by using resonance Raman spectroscopy in conjunction with the complete active space self-consistent field (CASSCF) method and density functional theory (DFT) calculations. The A-band resonance Raman spectra in acetonitrile, methanol, and water were obtained at 299.1, 282.4, 266.0, 252.7, and 245.9 nm excitation wavelengths to probe the structural dynamics of thioacetamide in the S2 state. CASSCF calculations were done to determine the transition energies and structures of the lower-lying excited states, the conical intersection points CI(S2/S1) and CI(S1/S0), and intersystem crossing points. The structural dynamics of thioacetamide in the S2 state was revealed to be along eight Franck-Condon active vibrational modes ν15, ν11, ν14, ν10, ν8, ν12, ν18, and ν19, mostly in the CC/CS/CN stretches and the CNH8,9/CCH5,6,7/CCN/CCS in-plane bends as indicated by the corresponding normal mode descriptions. The S2 → S1 decay process via the S2/S1 conical intersection point as the major channel were excluded. The thione-thiol photoisomerization reaction mechanism of thioacetamide via the S2,FC → S'1,min excited state proton transfer (ESPT) reaction channel was proposed.

Pygmy resonances and nucleosynthesis

A microscopic theoretical approach based on a self-consistent density functional theory for the nu- clear ground state and QRPA formalism extended with multi-phonon degrees of freedom for the nuclear excited states is implemented in investigations of new low-energy modes called pygmy resonances. Advantage of the method is the unified description of low-energy multiphonon excitations, pygmy resonances and core polariza- tion effects. This is found of crucial importance for the understanding of the fine structure of nuclear response functions at low energies. Aspects of the precise knowledge of nuclear response functions around the neutron threshold are discussed in a connection to nucleosynthesis.

Negative Differential Resistance and Spin-Filtering Effects in Zigzag Graphene Nanoribbons with Nitrogen-Vacancy Defects

We explore the electronic and transport properties of zigzag graphene nanoribbons (GNRs) with nitrogen-vacancy defects by performing fully self-consistent spin-polarized density functional theory calculations combined with non-equilibrium Green's function technique. We observe robust negative differential resistance (NDR) effect in all examined molecular junctions. Through analyzing the calculated electronic structures and the bias-dependent transmission coefficients, we find that the narrow density of states of electrodes and the bias-dependent effective coupling between the central molecular orbitals and the electrode subbands are responsible for the observed NDR phenomenon. In addition, the obvious difference of the transmission spectra of two spin channels is observed in some bias ranges, which leads to the near perfect spin-filtering effect. These theoretical findings imply that GNRs with nitrogen-vacancy defects hold great potential for building molecular devices.

Computing total energies in complex materials using charge self-consistent DFT + DMFT

We have formulated and implemented a fully charge self-consistent density functional theory plus dynamical mean-field theory methodology which enables an efficient calculation of the total energy of realistic correlated electron systems. The density functional portion of the calculation uses a planewave basis set within the projector augmented wave method enabling study of systems with large, complex unit cells. The dynamical mean-field portion of the calculation is formulated using maximally localized Wannier functions, enabling a convenient implementation which is independent of the basis set used in the density functional portion of the calculation. The importance of using a correct double-counting term is demonstrated. A generalized form of the standard double-counting correction, which we refer to as the ${U}^{\ensuremath{'}}$ form, is described in detail and used. For comparison, the density functional plus $U$ method is implemented within the same framework including the generalized double counting. The formalism is validated via a calculation of the metal-insulator and structural phase diagrams of the rare-earth nickelate perovskites as functions of applied pressure and A-site rare-earth ions. The calculated density functional plus dynamical mean-field results are found to be consistent with experiment. The density functional plus $U$ method is shown to grossly overestimate the tendency for bond disproportionation and insulating behavior.

Atomically thin heterostructures based on single-layer tungsten diselenide and graphene.

Heterogeneous engineering of two-dimensional layered materials, including metallic graphene and semiconducting transition metal dichalcogenides, presents an exciting opportunity to produce highly tunable electronic and optoelectronic systems. In order to engineer pristine layers and their interfaces, epitaxial growth of such heterostructures is required. We report the direct growth of crystalline, monolayer tungsten diselenide (WSe2) on epitaxial graphene (EG) grown from silicon carbide. Raman spectroscopy, photoluminescence, and scanning tunneling microscopy confirm high-quality WSe2 monolayers, whereas transmission electron microscopy shows an atomically sharp interface, and low energy electron diffraction confirms near perfect orientation between WSe2 and EG. Vertical transport measurements across the WSe2/EG heterostructure provides evidence that an additional barrier to carrier transport beyond the expected WSe2/EG band offset exists due to the interlayer gap, which is supported by theoretical local density of states (LDOS) calculations using self-consistent density functional theory (DFT) and nonequilibrium Green's function (NEGF).

Trends in Formic Acid Decomposition on Model Transition Metal Surfaces: A Density Functional Theory study

We present a first-principles, self-consistent periodic density functional theory (PW91-GGA) study of formic acid (HCOOH) decomposition on model (111) and (100) facets of eight fcc metals (Au, Ag, Cu, Pt, Pd, Ni, Ir, and Rh) and (0001) facets of four hcp (Co, Os, Ru, and Re) metals. The calculated binding energies of key formic acid decomposition intermediates including formate (HCOO), carboxyl (COOH), carbon monoxide (CO), water (H2O), carbon dioxide (CO2), hydroxyl (OH), carbon (C), oxygen (O), and hydrogen (H; H2) are presented. Using these energetics, we develop thermochemical potential energy diagrams for both the carboxyl-mediated and the formate-mediated dehydrogenation mechanisms on each surface. We evaluate the relative stability of COOH, HCOO, and other isomeric intermediates (i.e., CO + OH, CO2 + H, CO + O + H) on these surfaces. These results provide insights into formic acid decomposition selectivity (dehydrogenation versus dehydration), and in conjunction with calculated vibrational frequenc...

Ab-initio calculations of electronic, transport, and structural properties of boron phosphide

We present results from ab-initio, self-consistent density functional theory calculations of electronic and related properties of zinc blende boron phosphide (zb-BP). We employed a local density approximation potential and implemented the linear combination of atomic orbitals formalism. This technique follows the Bagayoko, Zhao, and Williams method, as enhanced by the work of Ekuma and Franklin. The results include electronic energy bands, densities of states, and effective masses. The calculated band gap of 2.02 eV, for the room temperature lattice constant of a = 4.5383 Å, is in excellent agreement with the experimental value of 2.02 ± 0.05 eV. Our result for the bulk modulus, 155.7 GPa, agrees with experiment (152–155 GPa). Our predictions for the equilibrium lattice constant and the corresponding band gap, for very low temperatures, are 4.5269 Å and 2.01 eV, respectively.