Density functional studies for the clean Fe(110) surface and for the adsorption of CO on Fe(110) at a half and a quarter monolayer CO coverage are reported. For the clean surface, we find only a modest surface relaxation and a modest increase of the magnetic moment of the first surface layer. At low CO coverage, CO adsorbs on top of Fe. But at half coverage, the density functional calculations predict that an off symmetry position between the hollow and bridge site results in the largest binding energy. Comparison of the vibrational frequencies with experiment however suggest, that the experimentally observed situation corresponds to atop adsorption at both coverage. Possible reasons for the discrepancy are discussed.
We have performed first-principles static and dynamic calculations based on density functional theory and the pseudopotential method to investigate the adsorption and deprotonation of methanol on the stoichiometric (110) surface of TiO2. Static calculations, employing full relaxation of adsorbate and substrate atom positions, are performed. In the high-coverage limit (θ = 1), we find that there are several structures of approximately equal stability. In two of these, the methanol molecule is dissociated, resulting from scission of the O−H or C−O bonds. In the third, methanol is molecularly adsorbed. Other structures of approximately equivalent energy contain 1:1 mixtures of these conformations. At lower coverage (θ = 1/2), we find that the two dissociative modes of adsorption found at θ = 1 are favored over molecular adsorption by 19 kJ/mol (O−H scission) and 7 kJ/mol (C−O scission). The adsorption energy of the most stable θ = 1/2 conformation changes by approximately ±5% as the coverage is reduced to θ = 1/3 and θ = 1/4. Intermolecular attractions and repulsions are found to play a crucial role in determining the stability of different conformations at different coverages. Conversion of the metastable θ = 1/2 molecularly adsorbed complex via O−H scission to a dissociated complex is predicted to be barrierless. First-principles molecular dynamics calculations on this system in which the methanol molecule approaches the surface predict spontaneous dissociation by rupture of the O−H bond and also that C−O bond breaking is likely to be an activated process. Further dynamical simulations indicate that the probability of finding conformations other than that obtained after O−H bond rupture is small.
We present ab-initio local-density functional calculations of the atomic reconstruction of clean and hydrogen-covered three dangling-bond (3db) diamond (111) surfaces, extending our earlier work on the one dangling-bond surface. The calculations are based on a finite-temperature local-density approximation, optimized ultrasoft pseudopotentials, and an exact calculation of the electronic ground state and Hellmann-Feynman forces before any step in the geometrical optimization of the surface. For the bulk-terminated surfaces we predict a difference of 1.90 eV per atom in the cleavage energies of the 1db and 3db surfaces (also referred to as shuffle- and glide-plane cleavage), i.e. considerably less than expected from a simple bond-scission argument. This difference is further reduced by reconstruction. We find that the clean 3db-C(111) surface reconstructs in a (2 × 1) geometry with symmetric, buckled π-bonded chains (Seiwatz chains) and a pronounced buckling in some of the deeper layers. However, a (√3 × √3) reconstruction with the surface atoms forming slightly buckled trimers is energetically less favourable by only 0.13 eV per atom. Hydrogenation of the surface stabilizes the (2 × 1) geometry of the surface relative to the (√3 × √3) geometry. Deposition of a monolayer of hydrogen reduces the buckling at the surface but does not change the single-chain topology. A hydrogen coverage of two H atoms per surface C atom leads to the formation of parallel rows of C2H4 units arranged again in a (2 × 1) geometry. A coverage of three H atoms per C atom leads to a complete de-reconstruction and stabilizes the 3db-C(111)-(1 × 1) surface (which is identical to a 1db-C(111)-(1 × 1) surface covered with CH3 groups). Our calculations show that at larger values of the hydrogen chemical potential, the strongly hydrogenated 3db surfaces are stabilized over the 1db surface.
Density functional calculations are carried out for the (0001) surface of V2O3 in the corundum structure. In thermal equilibrium with the bulk, the dominant surface termination is characterised by vanadyl (VO) groups adsorbed on the (0001) surface. Under increased oxygen pressure, the calculations predict that a pure oxygen termination is stable, whereas under very oxygen poor conditions the removal of the oxygen from the VO group can result in a stoichiometric metal termination. The electronic states in the valence band regime, the oxygen and vanadium core-level binding energies, and the vibrational spectra are calculated for the bulk and the surface by density functional theory and compared to recent experimental studies.
The structural and electronic properties of clean and hydrogenated diamond (100) surfaces have been investigated via fully self-consistent ab initio calculations. We find that the clean and the monohydrogenated surfaces reconstruct via the formation of rows of symmetric dimers. Further hydrogenation to a coverage of 1.5 monolayers stabilizes a surface with a (1 × 1) periodicity in the C layers, albeit with a low H-desorption energy for the formation of the reconstructed monohydride surface. The electronic properties of the surfaces have been calculated. Surface states within the bulk gap are found for the clean surface and for the strongly hydrogenated surface, but not in the monohydride case.
We extend the full-potential projector-augmented-wave method beyond the frozen core approximation, i.e., include the self-consistent optimization of the core charge density, in such a manner that the valence wave functions remain orthogonal to the core. The method consists of an on-the-fly repseudization of the all-electron problem, solving for the self-consistent core charge density within a spherical approximation. The key ideas in our procedure are to keep the projector functions fixed throughout the electronic minimization and to derive the new pseudopartial waves from these original projector functions, at each step of the electronic minimization procedure. Results of relaxed core calculations for atomic interconfigurational energies, structural energy differences between bulk phases of Fe, atomization energies of a subset of Pople's G2-1 set, and the Rh 3d surface core level shifts for the (log3 x log3)-Rh(111) surface at 1/3 CO coverage are presented.
We present ab initio investigations of the structural and electronic properties of clean and hydrogen-covered diamond (100) surfaces within local-density-functional theory. Our calculations are based on a variational solution of the Kohn-Sham equations using a preconditioned conjugate-gradient approach and on the optimization of the atomic structure via a quasi-Newton quench based on the exact Hellmann-Feynman forces. The computations are performed in a plane-wave basis, the electron-ion interaction is described by optimized ultrasoft pseudopotentials. We find that the clean and the monolayer-covered surfaces reconstruct in a (2\ifmmode\times\else\texttimes\fi{}1) cell via the formation of rows of symmetric \ensuremath{\pi}-bonded dimers. Further hydrogenation to a coverage of 1.5 ML stabilizes a surface with a (1\ifmmode\times\else\texttimes\fi{}1) periodicity in the C layers, albeit with a low H-desorption energy for the formation of the reconstructed monohydride surface. The two-step desorption process is in good agreement with experimental observations. Electronic surface states within the bulk gap are predicted for the clean surface, but not for the monohydride case. The detailed analysis of the layer-resolved local densities of states and of the dispersion of the surface states demonstrates that the results are in good agreement with recent photoemission experiments. A negative electron affinity is predicted for the monohydride surface, but not for the clean surface. \textcopyright{} 1996 The American Physical Society.
No abstract is provided for this article.
Recent measurements have shown that the inorganic clathrate Sr8Ga16Ge30 has good thermoelectric properties. This discovery has caused intense experimental activity to synthesize and test other compounds in this class. It has been conjectured that clathrates may be good thermoelectrics if they satisfy several conditions. The Sr atoms, trapped inside the clathrate cages, scatter phonons efficiently, leading to low thermal conductivity. Electric conductivity takes place mostly through the clathrate frame and the conduction electrons are not scattered by Sr vibrations. The compounds, being made of atoms that are semiconductors in the solid state, may have a high Seebeck coefficient. There has been no direct evidence, experimental or theoretical, for this scenario. By performing density functional calculations we show that these ideas are correct. The Sr atoms are weakly bound to the cage and do undergo large-amplitude motion. An analysis of conductivity shows that the largest contribution comes from a band in which the electrons are located on the clathrate frame. Bands originating from the Sr atoms contribute little to conductivity. There is very little charge transfer between the Sr atoms and the frame, and as a result, Sr vibrations are weakly coupled to the conduction electrons. The calculated Seebeck coefficient is in reasonable agreement with the measured one. We find that it is strongly affected by the positions of the Ga atoms in the frame and by doping.
Constructing a self-consistent first-principles framework that accurately predicts the properties of electron transfer reactions through finite-temperature molecular dynamics simulations is a dream of theoretical electrochemists and physical chemists. Yet, predicting even the absolute standard hydrogen electrode potential, the most fundamental reference for electrode potentials, proves to be extremely challenging. Here, we show that a hybrid functional incorporating 25 % exact exchange enables quantitative predictions when statistically accurate phase-space sampling is achieved via thermodynamic integrations and thermodynamic perturbation theory calculations, utilizing machine-learned force fields and $\Delta$-machine learning models. The application to seven redox couples, including molecules and transition metal ions, demonstrates that the hybrid functional can predict redox potentials across a wide range of potentials with an average error of 80 mV.
The structural properties of the compound ${\mathrm{YSi}}_{2}$ are investigated by means of ab initio simulations based on density-functional theory. More particularly we emphasize the role played by Si vacancies, show that the ${\mathrm{Th}}_{3}$${\mathrm{Pd}}_{5}$ structure can be deduced from the ${\mathrm{AlB}}_{2}$ structure by a relaxation process around the Si vacancies within the (0001) plane. A specific ordered arrangement of the Si vacancies along the [0001] direction is found to be energetically the most favorable. Geometries obtained from our theoretical calculations are in good agreement with experimental results.
We present $GW$ calculations for small and large gap systems comprising typical semiconductors (Si, SiC, GaAs, GaN, ZnO, ZnS, CdS, and AlP), small gap semiconductors (PbS, PbSe, and PbTe), insulators (C, BN, MgO, and LiF), and noble gas solids (Ar and Ne). It is shown that the ${G}_{0}{W}_{0}$ approximation always yields too small band gaps. To improve agreement with experiment, the eigenvalues in the Green's function $G$ $(G{W}_{0})$ and in the Green's function and the dielectric matrix $(GW)$ are updated until self-consistency is reached. The first approximation leads to excellent agreement with experiment, whereas an update of the eigenvalues in $G$ and $W$ gives too large band gaps for virtually all materials. From a pragmatic point of view, the $G{W}_{0}$ approximation thus seems to be an accurate and still reasonably fast method for predicting quasiparticle energies in simple $sp$-bonded systems. We furthermore observe that the band gaps in materials with shallow $d$ states (GaAs, GaN, and ZnO) are systematically underestimated. We propose that an inaccurate description of the static dielectric properties of these materials is responsible for the underestimation of the band gaps in $G{W}_{0}$, which is itself a result of the incomplete cancellation of the Hartree self-energy within the $d$ shell by local or gradient corrected density functionals.
First-principles calculations for tetragonal zirconia are presented. The stability of the terminations is investigated for different setups (symmetrically or asymmetrically terminated or metal supported) and as function of the film thickness. Several aspects, which are of importance for polar surfaces, are discussed on the basis of the results from the asymmetric setup. Among other things the dielectric constant can be derived from the slab calculations. For the more stable (101) surface a surface phase diagram under hydrogen and oxygen atmosphere is presented. Finally, the interaction between yttrium dopants and the surface is analyzed. While such defects prefer positions far from the surface for a (001) oriented surface, the more stable (101) surface becomes even more stabilized by defects close to the surface. This result is in agreement with experimental findings.
