Modern computing facilities grant access to first-principles density-functional theory study of complex physical and chemical phenomena in materials, that require large supercell to properly model the system. However, supercells are associated to small Brillouin zones in the reciprocal space, leading to folded electronic eigenstates that make the analysis and interpretation extremely challenging. Various techniques have been proposed and developed in order to reconstruct the electronic band structures of super cells, unfolded into the reciprocal space of an ideal primitive cell. Here, we propose an efficient unfolding scheme embedded directly in the Vienna Ab-initio Simulation Package (VASP), that requires modest computational resources and allows for an automatized mapping from the reciprocal space of the supercell to primitive cell Brillouin zone. This algorithm can computes band structures, Fermi surfaces and spectral functions, by using an integrated post-processing tool (bands4vasp). The method is here applied to a selected variety of complex physical situations: the effect of doping on the band dispersion in the BaFe$_{\rm 2(1-x)}$Ru$_{\rm 2x}$As$_2$ superconductor, the interaction between adsorbates and polaronic states on the TiO$_2$(110) surface, and the band splitting induced by non-collinear spin fluctuations in EuCd$_2$As$_2$.
The ionization potential is a fundamental key quantity with great relevance to diverse material properties. We find that state of the art methods based on density functional theory and simple diagrammatic approaches as commonly taken in the $GW$ approximation predict the ionization potentials of semiconductors and insulators unsatisfactorily. Good agreement between theory and experiment is obtained only when diagrams resulting from the antisymmetry of the many-electron wave function are taken into account via vertex corrections in the self-energy. The present approach describes both localized and delocalized states accurately, making it ideally suited for a wide class of materials and processes.
We present ab initio local-density functional calculations of the electronic structure of clean and hydrogen-covered three-dangling-bond (3db) diamond (111) surfaces, extending our earlier work on the atomic structure of the three-dangling-bond surface. Our results show that the three photoelectron spectra measured on the C(111) surface at different degrees of hydrogenation can be attributed to the 1db-(2 × 1), 1db-(1 × 1):H and 3db-(2 × 1):2H, in agreement with the predicted stabilities as a function of the chemical potential of hydrogen above the surface.
The atomic and magnetic structure of FCC-Fe layers on a Cu(100) substrate is investigated by ab initio calculations as a function of the thickness of the film. Fe films with a thickness varying between 1 and 3ML stabilize a ferromagnetic (FM) ground state. For thicker films with an even number of Fe layers (4, 6 and 8ML) a bilayer AF magnetic structure develops, while for an odd number of Fe layers (5, 7 and 9ML) various spin structures coexist. The magnetic properties of Fe thin layers are compared to those of bulk FCC-Fe biaxially strained to the lattice constant of the Cu substrate and the dependence of magnetic energies in the form of the exchange-correlation functional is discussed.
For optimal accuracy, auxiliary-field quantum Monte Carlo (AFQMC) requires trial states consisting of multiple Slater determinants. We develop an efficient algorithm to select the determinants from an AFQMC random walk eliminating the need for other methods. When determinants contribute significantly to the non-orthogonal configuration interaction energy, we include them in the trial state. These refined trial wave functions significantly reduce the phaseless bias and sampling variance of the local energy estimator. With 100 to 200 determinants, we lower the error of AFQMC by up to a factor of 10 for second row elements that are not accurately described with a Hartree-Fock trial wave function. For the HEAT set, we improve the average error to within the chemical accuracy. For benzene, the largest studied system, we reduce AFQMC error by 80% with 214 Slater determinants and find a 10-fold increase of the time to solution. We show that the remaining error of the method prevails in systems with static correlation or strong spin contamination.
The oxidation of Pd(111) leads to an incommensurate surface oxide, which was studied by the use of scanning tunneling microscopy, surface x-ray diffraction, high resolution core level spectroscopy, and density functional calculations. A combination of these methods reveals a two-dimensional structure having no resemblance to bulk oxides of Pd. Our study also demonstrates how the atomic arrangement of a nontrivial incommensurate surface can be solved by molecular dynamics in a case where experimental techniques alone give no solution.
We present ab-initio local-density-functional investigations of clean and hudrogen-covered one- (1db) and three-dangling-bond (3db) diamond (111) surfaces, the geometries of the reconstructed surfaces and their stabilities at different values of the hydrogen chemical potential. At low values of the hydrogen chemical potential the clean 1db–(2 × 1) π-bonded Pandey-chain structure forms the ground state. As the hydrogen chemical potential increases, first the hydrogenated 1db–(1 × 1):H structure becomes stable. Then we find a narrow region where a 3db–(2 × 1):2H dihydride surface is stable, until at last a fully hydrogen-saturated 3db–(1 × 1):3H surface has the lowest energy. Then preliminary molecular-dynamics results of reconstruction and graphitization for the 1db surface at elevated temperatures are reported.
We present an implementation of the Bethe-Salpeter equation (BSE) for core-conduction band pairs within the framework of the projector augmented-wave method. For validation, the method is applied to the $K$-edges of diamond, graphite, hexagonal boron-nitride, as well as four lithium-halides (LiF, LiCl, LiI, LiBr). We compare our results with experiment, previous theoretical BSE results, and the density functional theory-based supercell core-hole method. In all considered cases, the agreement with experiment is excellent, in particular for the position of the peaks as well as the fine structure. Comparing BSE to supercell core-hole spectra we find that the latter often qualitatively reproduces the experimental spectrum, however, it sometimes lacks important details. This is shown for the $K$-edges of diamond and nitrogen in hexagonal boron-nitride, where we are capable to resolve within the BSE experimental features that are lacking in the core-hole method. Additionally, we show that in certain systems the supercell core-hole method performs better if the excited electron is added to the background charge. We attribute this improved performance to a reduced self-interaction.
By performing accurate ab-initio density functional theory calculations, we study the role of $4f$ electrons in stabilizing the magnetic-field-induced ferroelectric state of DyFeO$_{3}$. We confirm that the ferroelectric polarization is driven by an exchange-strictive mechanism, working between adjacent spin-polarized Fe and Dy layers, as suggested by Y. Tokunaga [Phys. Rev. Lett, \textbf{101}, 097205 (2008)]. A careful electronic structure analysis suggests that coupling between Dy and Fe spin sublattices is mediated by Dy-$d$ and O-$2p$ hybridization. Our results are robust with respect to the different computational schemes used for $d$ and $f$ localized states, such as the DFT+$U$ method, the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional and the GW approach. Our findings indicate that the interaction between the $f$ and $d$ sublattice might be used to tailor ferroelectric and magnetic properties of multiferroic compounds.
In the present work, we apply an ab initio approach to study the dissociative adsorption of a hydrogen molecule on the palladium (111) surface. Our approach is based on the density functional theory (DFT) with a generalized gradient approximation (GGA). The wave-functions are expanded in a plane-wave basis and the electron-ion interaction is described by ultrasoft pseudopotentials. Several pathways for the dissociative adsorption are studied in details. The potential energy surface for each path is determined. Spontaneous dissociation paths are found which have a very small barrier of about 70 meV. This agrees fairly well with the estimation based on the molecular beam experiments, 50 meV. We find that the geometric factor plays an important role: larger stretching of the hydrogen molecule bond leads to a higher barrier. Our results give also some indication for the existence of a precursor state.
In this report, we present the detailed analysis of the interstitial oxygen (O2+, O0, O2-) diffusion in monoclinic HfO2 (hafnia) using the first principles calculations. The interstitial oxygen atom kicks out the oxygen atom at the 3-fold-site and occupies the 3-fold-site. And then the newly kicked-out interstitial oxygen atom jumps to the nearest neighbor site and couples again with the atoms at the crystal sites. This kick-out- mechanism is valid for all charge states of the interstitial oxygen in monoclinic HfO2. In hafnia, the interstitial oxygen atom can take 3 charge states (+2, 0, -2) depending on the chemical potential (Ef), whereas the oxygen-vacancy in hafnia can get +2 or 0 charge state being dependent on Ef. In the lower range of Ef, O2+ and O0 might contribute. In the middle range of Ef, the O2- does not contribute to the diffusion process in hafnia because of the pair annihilation process between O2- and oxygen vacancy (V2+) defect pair. We can simulate such a pair annihilation process in hafnia. In the higher range, O2- might contribute the diffusion process.
Determining the finite-temperature structure of the hybrid perovskite MAPbI3 is a challenge for both experimental and theoretical methods. A very powerful computational method that can resolve the atomic structure is molecular dynamics (MD). The resulting structure depends on the density functional approximation (DFA) in the case of ab initio MD and the force field in classical MD. We compare the structure between 250 and 400 K obtained with different DFAs and force fields in one consistent manner. The symmetry of the PbI3 framework is analyzed as well as the relative ordering of the neighboring organic molecules inside the framework. The distribution function of the molecules is used to map out an effective energy surface for the rotation of a single molecule. This surface is accurately modeled by a pair of cubic harmonics. Available experimental data in literature are discussed and compared to the structure obtained with the different methods. The spread in these data is still too large to uniquely determine the method that “best” describes the perovskite, however, promising candidates and outliers have been identified.
