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First-principles self-consistent spin-polarized electronic structure calculations were performed for the nanoscale magnetic molecules Mn12O12(CH3COO)16(H2O)4 and Fe11O6(OH)6(O2CPh)15 . The numerical Discrete Variational method was employed, within Density Functional theory. Charges and magnetic moments were obtained for the atoms, as well as Density of States diagrams, and charge and spin density maps. For Mn12O12(CH3COO)16(H2O)4 , values of the Heisenberg exchange parameters J were derived from the calculations; Mossbauer hyperfine parameters were calculated for Fe11O6(OH)6(O2CPh)15 and compared to reported experimental values. Key-words: Nanoscale; Molecular magnets.

The substitution of Ni by the transition metals Fe, Co, and Ru in YNi{sub 2}B{sub 2}C is analyzed using the self-consistent local-density embedded-cluster approach. Changes of partial densities of states, charge density, and bond order are examined to elucidate bonding structure and the interplay between covalent, ionic, and metallic interactions in these highly anisotropic superconductors. Impurity induced modifications in distribution and composition of states near the Fermi level are discussed in terms of the observed drop in {ital T}{sub {ital c}} with impurity concentration. Electric-field gradients are obtained at the central site. In the case of iron substitution with concentration {ital x}{le}0.1, electric quadrupole splitting is compared with M{umlt o}ssbauer experimental results. {copyright} {ital 1996 The American Physical Society.}

Tendencies toward local atomic ordering in (A,A′)(B,B′)O_(3−δ) mixed composition perovskites are modeled to explore their influence on thermodynamic, transport, and electronic properties. In particular, dopants and defects within lanthanum chromate perovskites are studied under various simulated redox environments. (La_(1−x),Sr_x)(Cr_(1−y),Fe_y)O_(3−δ) (LSCF) and (La_(1−x),Sr_x)(Cr_(1−y),Ru_y)O_(3−δ) (LSCR) are modeled using a cluster expansion statistical thermodynamics method built upon a density functional theory database of structural energies. The cluster expansions are utilized in lattice Monte Carlo simulations to compute the ordering of Sr and Fe(Ru) dopant and oxygen vacancies (Vac). Reduction processes are modeled via the introduction of oxygen vacancies, effectively forcing excess electronic charge onto remaining atoms. LSCR shows increasingly extended Ru-Vac associates and short-range Ru-Ru and Ru-Vac interactions upon reduction; LSCF shows long-range Fe-Fe and Fe-Vac interaction ordering, inhibiting mobility. First principles density functional calculations suggest that Ru-Vac associates significantly decrease the activation energy of Ru-Cr swaps in reduced LSCR. These results are discussed in view of experimentally observed extrusion of metallic Ru from LSCR nanoparticles under reducing conditions at elevated temperature.

Self-consistent calculations with the discrete variational method in the framework of local spin-density theory were performed for 43-atom embedded clusters representing a Mn impurity in Al and Cu hosts. Effects of local lattice relaxation were explored by varying the distance between Mn and the nearest-neighbor host atoms. It was found that the magnetic moment of Mn in Al is much more sensitive to local lattice distortion than Mn in Cu. At the equilibrium Mn nearest-neighbors distance, as determined by x-ray absorption fine-structure measurements, a reduction of 34% of \ensuremath{\mu} in Al is obtained, relative to the host lattice distance. Mechanisms related to the effect of the environment on \ensuremath{\mu} are discussed. The Mn 3s exchange splitting observed in x-ray photoelectron spectra is also investigated, and it is found that it may not be taken as evidence of a 3d moment in the ground state.

Electronic structure and energetics of (0001) and (11\ifmmode\bar\else\textasciimacron\fi{}02) surfaces of sapphire, \ensuremath{\alpha}-${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$, were calculated using the self-consistent-field discrete variational (DV) method in the local-density framework. Clusters of a size of 70--120 atoms embedded in the semi-infinite host lattice were used to model the sapphire free surfaces. Calculations were performed on all possible terminating (0001) and (11\ifmmode\bar\else\textasciimacron\fi{}02) surfaces obtained from cleaving a sapphire single crystal. The energetic calculations show that the surface with the lowest cleavage energy is terminated with an Al layer for the (0001) surface, while it is terminated with an O layer for the (11\ifmmode\bar\else\textasciimacron\fi{}02) surface. The concept of the surface building block, useful in determining the surface atomic termination, was proposed. Two unoccupied surface bands derived from the top-layer Al atoms are found within the bulk band gap at 2 and 8 eV below the conduction-band minimum (CBM) for the (0001) surface. For the (11\ifmmode\bar\else\textasciimacron\fi{}02) surface, the occupied valence states derived from 2p states of the top-layer O atoms are found \ensuremath{\sim}4.0 eV below the CBM, which may explain the 4.7-eV energy-loss feature found experimentally on a sapphire (11\ifmmode\bar\else\textasciimacron\fi{}02) surface.

First-principles electronic-structure calculations were performed for embedded clusters representing an Fe impurity in the metal hosts Be, Mg, Ca, Sr, and Al. The discrete-variational method was employed in the framework of local-spin-density theory. BeFe was found to be nonmagnetic, as well as AlFe, this last if local relaxation is taken into account. For Fe in the other alkaline-earth metals, local magnetic moments g2${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ were encountered. Mechanisms leading to moment stability or moment quenching are discussed. The contact hyperfine field at the Fe nucleus in CaFe was calculated, and a small positive value was found. Magnetic susceptibilities were estimated employing the virtual-bound-state model, with parameters derived from the calculations.

We present the results of ab initio hybrid density-functional calculations of the atomic and the electronic structures of ${\text{LaMnO}}_{3}$ (LMO) and ${\text{La}}_{1\ensuremath{-}{x}_{b}}{\text{Sr}}_{{x}_{b}}{\text{MnO}}_{3}$ (001) surfaces. The total energies obtained from these calculations were used to analyze thermodynamic stability of the surfaces. We predict Sr and O vacancy segregation to the surface to occur with similar energies ($\ensuremath{\sim}0.5\text{ }\text{eV}$ per defect). In pure LMO only ${\text{MnO}}_{2}$ termination is thermodynamically favorable under typical operational conditions of a cathode in solid oxide fuel cells, whereas Sr doping makes La(Sr)O termination favorable. Finally, the role of Sr doping in cathode degradation is discussed.

The polarized copper {ital K}-edge x-ray-absorption near-edge spectra (XANES) in the high-{Tc} superconductor YBa{sub 2}Cu{sub 3}O{sub 7} and related semiconductor oxide YBa{sub 2}Cu{sub 3}O{sub 6} were calculated. The initial states and the final bound states were obtained from the self-consistent-field discrete-variational {ital X}{alpha} method (SCF DV-{ital X}{alpha}). Continuum states were found by the multiple-scattering method using the muffin-tin truncation of the SCF DV-{ital X}{alpha} potentials. XANES for each of the two Cu sites in these compounds were identified by using the calculated difference in Cu 1{ital s} ionization potentials. Features in XANES were interpreted with the help of the projected density of states and photoelectron trapping times. A quantitative comparison with the measured spectra leads us to assign shakedown features with relaxation energy of 8.3 eV and intensities of 20% and 12% of the total transition in the {ital a}-axis- and {ital b}-axis-polarized Cu {ital K}-edge XANES of YBa{sub 2}Cu{sub 3}O{sub 7}, respectively. This shakedown feature apparently involves ligand{r arrow}3{ital d} transitions in the core-hole excited state. It is found that this relaxation process is present in the metallic YBa{sub 2}Cu{sub 3}O{sub 7} and absent in the semiconducting phase.

We report the results of theoretical calculations of copper K-edge x-ray-absorption near-edge spectra (XANES) in ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ and related oxides ${\mathrm{Cu}}_{2}$O, CuO, and ${\mathrm{KCuO}}_{2}$. The final bound states were obtained from the self-consistent-field discrete-variational X\ensuremath{\alpha} method (SCF DV X\ensuremath{\alpha}), and continuum states were found by the multiple-scattering method using the muffin-tin truncation of the SCF DV X\ensuremath{\alpha} potentials. Composition of the final-state wave functions was analyzed. The 1s ionization potentials of the three reference compounds obtained from the SCF DV X\ensuremath{\alpha} transition-state calculations were used to set up the relative energy scale for the calculated cross section. The principal features of measured Cu K-edge XANES for ${\mathrm{Cu}}_{2}$O, CuO, and ${\mathrm{KCuO}}_{2}$ were reproduced satisfactorily by our calculations along with their relative energy positions. Our calculated polarized XANES of ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ were compared with measured spectra, and two shakeup features were identified with radiation polarized along the crystal c axis. By quantitatively comparing the measured spectra with a model based on our calculated cross sections, their intensities were found to be \ensuremath{\sim}24% and 10% of the main transition with shakeup energies of 5.7 and 9.5 eV, respectively. We suggest that these multielectron excitation features involve Cu 3d\ensuremath{\rightarrow}4p transitions.

We present the results of ab initio calculations on magnetic and electronic structures of La1�xSrxMnO3 at low doping, x =1/8. Using the B3LYP hybrid exchange-correlation functional within the framework of densityfunctional theory, we predict a ferromagnetic ground state for La0.875Sr0.125MnO3 in both the low-temperature orthorhombic and the high-temperature pseudocubic phases. This is in contrast to its parent compound LaMnO3, for which we find in agreement with experiment the layered antiferromagnetic state to be the most stable one. The calculated density of states and bond population analysis suggest a tendency of formation of half-metallic spin states in the band gap of both structures.

Using the B3PW hybrid exchange-correlation functional within density-functional theory and employing Gaussian-type basis sets, we calculated the atomic and electronic structures and thermodynamic stability of three double-layered (DL) SrTiO3(001) surfaces: (i) SrO-terminated, (ii) TiO2-terminated, and (iii) (2×1) reconstruction of TiO2-terminated SrTiO3(001) recently suggested by Erdman et al. [Nature (London) 419, 55 (2002)]. A thermodynamic stability diagram obtained from first-principles calculations shows that regular TiO2- and SrO-terminated surfaces are the most stable. The stability regions of (2×1) DL TiO2- and DL SrO-terminated surfaces lie beyond the precipitation lines of SrO and TiO2 compounds and thus are less stable with respect to regular SrTiO3(001) surfaces. Analysis of the stability diagram suggests that Sr precipitation on SrTiO3 surface never occurs. Our simulations show a substantial increase of Ti-O covalency on the DL surfaces as compared to the regular surfaces, which are themselves more covalent than the crystalline bulk. The implications of our calculated results for recent experimental observations are discussed.

Gray tin films enriched by over 95% ${}^{119}\mathrm{Sn}$ and grown by molecular beam epitaxy on CdTe(001) wafers are characterized by inelastic nuclear resonance spectroscopy and investigated theoretically by embedded-cluster density-functional theory methods. Experimental tin phonon densities of states are obtained via analysis of resonant scattering of the 23.88-keV nuclear transition, making use of a high-resolution spectrometer at the Advanced Photon Source. Conventional M\"ossbauer spectroscopy is used in the scattering mode to determine hyperfine parameters of the \ensuremath{\alpha}-Sn phase and, after thermal treatment, the \ensuremath{\beta} phase. Electronic structure in the vicinity of Sn-Cd and Sn-Te interfaces is calculated in order to determine local charge transfer and changes in hyperfine parameters for ${}^{119}\mathrm{Sn}$ atoms in the interface region. Although, due to sample thickness, both experiments reveal properties essentially of the bulk, the calculations allow investigation of surface and interface regions at an atomic level, thus providing complementary information. Effects of interlayer relaxation are explored.

Electron paramagnetic resonance, Mossbauer spectroscopy, and electronic structure calculations were combined in order to study the local geometry of Fe 2 + /Fe 3 + in Fe-doped hydroxyapatite. Atomistic simulations were carried out to obtain estimates of local geometry and lattice strain associated with fourfold, fivefold, and sixfold Fe sites. First-principles embedded cluster density functional calculations were performed to investigate the electronic structure associated with the substitution of calcium by Fe 2 + /Fe 3 + . Mossbauer isomer shift, quadrupole splitting, and the hyperfine magnetic field were calculated for each site and local coordination, for comparison to an experimental fit to a five-line model consisting of two bulk sites each for Fe 2 + and Fe 3 + and a surface hematitelike Fe 3 + species.

Nonrelativistic and four-component relativistic electronic structure calculations with the discrete variational method within density functional theory were performed for embedded clusters representing the layered compound ${\mathrm{EuCo}}_{2}{\mathrm{P}}_{2},$ with ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure. A relativistic one-electron model based on Kramers degeneracy was devised to describe the observed changes in the magnetism of this compound induced by applied pressure, which include the collapse of the Eu moment and the formation of Co moments. The ${}^{151}\mathrm{Eu}$ M\"ossbauer isomer shift calculated with relativistic wave functions agrees well with experiment; however, the ${\mathrm{Eu}}^{2+}{\ensuremath{-}\mathrm{E}\mathrm{u}}^{3+}$ valence change previously proposed is not predicted.

Energy bands of the "covalent metal" La${\mathrm{B}}_{6}$ have been calculated by a discrete variational method in the Hartree-Fock-Slater model. We find that the basic topology of the bands and the predicted Fermi surface are rather insensitive to the atomic configuration assumed in constructing the potential. The proposed Fermi surface is consistent with the basic features of published experimental de Haas-van Alphen data; it is not consistent with the hypothesis of magnetic breakdown which has been invoked to explain the finer details. A study of selected valence-band and conduction-band wave functions supports the conclusion that La-B bonding is more important than La-La bonding in explaining the metallic behavior of La${\mathrm{B}}_{6}$. The total density of states and the interband joint density of states for several bands are calculated and the results are compared to x-ray and optical data. Preliminary calculations of the energy bands of Y${\mathrm{B}}_{6}$ are presented; the similarity of these bands to those of La${\mathrm{B}}_{6}$ indicates that the relatively high superconducting transition temperature of Y${\mathrm{B}}_{6}$ cannot be explained by simple density-of-states arguments.

The measurement of Cu K-edge x-ray absorption near-edge structure of monovalent, divalent, and trivalent copper oxides has been used to identify the presence of different valence of copper in the La/sub 2-//sub x/(Sr,Ba)/sub x/CuO/sub 4/ (x = 0.0--0.3) system. The results indicate the coexistence of Cu II and Cu III states in these compounds. Theoretical calculations based on a cluster of CuO/sub 4/O/sub 2/La/sub 8/LaSr predict the presence of nearly degenerate electronic ground states representing nominally Cu II and Cu III configurations.

The electronic, magnetic, and hyperfine properties of 15-atom clusters of Fe/V superstructures are calculated using the first-principles discrete-variational X-italic..cap alpha.. method (DV-X-italic..cap alpha..). The superstructures include a monolayer of Fe sandwiched between two thick V layers, an Fe/V interface, and a single layer of Fe on bulk V. The numbers of V atoms in the first and second shells surrounding the central Fe atom (N-italic and M-italic, respectively) were varied and the trends exhibited by the electronic magnetic moments, ..mu.., hyperfine fields, H-italic/sub c-italic/, isomer shifts, IS, and the electric quadrupole splitting, ..delta../sub EQ/, were investigated. The results show that the magnetic moment on the central Fe atom does not decrease with increasing N-italic contrary to previous interpretations of experimental results. The dependence of the central atom magnetic moment on M-italic is minor. The magnetic hyperfine field decreases in magnitude with increasing N-italic and almost vanishes when N-italic = 8. The 3d-italic partial electronic density of states reflects asymmetry in the bonding between the spin-up and the spin-down states of Fe and V and the asymmetry becomes more pronounced with increasing N-italic. The isomer shift of Fe/V systems relative to ..cap alpha..-Fe becomes more negative as N-italic increases. A linearmore » correlation of the average H-italic/sub c-italic/ and IS is found and its slope determined. It is indicated that this slope could be used to determine ..cap alpha.., the isomer-shift calibration constant. The electric quadrupole splitting ..delta../sub EQ/ shows no regular trends. A model is proposed to explain the above trends.« less

The electronic structure and stabilization energy of isolated defects and defect clusters in wustite ${\mathrm{Fe}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$O is studied by the first-principles local-density theory. The embedded-cluster model is used to obtain linear combination of atomic orbitals expansions of one-electron properties in the discrete-variational scheme. The stability of the 4:1 interstitial cluster relative to simple defects is verified, and the tendency of 4:1 clusters to aggregate is explored. Open edge- and vertex-shared aggregates which permit the close approach of charge-compensating ${\mathrm{Fe}}^{3+}$ ions are found to be energetically favored.

The equilibrium geometry, binding energy, and electronic structure of small metal particles are investigated using self-consistent one-electron local-density theory. Results for ${\mathrm{Cu}}_{2}$, ${\mathrm{Cu}}_{4}$, and fcc ${\mathrm{Cu}}_{13}$ and ${\mathrm{Cu}}_{79}$ clusters show an increasing equilibrium bond length with cluster size, and a stiffening of the ${a}_{1}$ vibrational force constants. The calculated binding energies of 1.05 (${\mathrm{Cu}}_{2}$), 1.26 (${\mathrm{Cu}}_{4}$), 2.19 (${\mathrm{Cu}}_{13}$), and 3.03 (${\mathrm{Cu}}_{79}$) eV/atom compare well with the experimental values of 1.00 (${\mathrm{Cu}}_{2}$) and 3.50 (bulk) eV/atom. For ${\mathrm{Cu}}_{2}$ the theoretical bond length and vibrational frequency are found to be in good agreement with experiment. Densities of states and core-level shifts are analyzed to display cluster-size effects. Charge-density maps are used to display the buildup of metallic bonding charge with increasing particle size.

The electronic structure of some rare-earth orthovanadates, RVO/sub 4/, where R = La, Ce, Nd, Eu, Tb, Dy, or Yb, has been studied using the discrete variational X..cap alpha.. method. Charge and spin densities and the derived potentials for clusters embedded in the crystal lattice were iterated to self-consistency with the use of the self-consistent charge procedure. The results are compared with experimental x-ray photoelectron spectra (XPS) and atomic calculations. Eigenvalues (or transition-state energies) are compared with XPS binding energies and the calculated exchange splittings of lanthanide 4s, 5s, and 4f levels are compared with experiment. For 4f 's, the exchange splitting between the least-bound spin-up and spin-down states is measured. An analysis of optical and luminescent properties is made based on the cluster calculations. The results show that the lanthanide 4f orbitals play an active role in bonding, and that luminescence can be related to a resonant energy transfer from excited vanadate orbitals to the lanthanide 4f states.

The M-edge x-ray-absorption near-edge spectra was calculated for bulk ${\mathrm{UO}}_{2}$ and gas-phase ${\mathrm{UCl}}_{4}$ from a multiple-scattering approach within the muffin-tin--potential approximation. The muffin-tin potential was constructed from the self-consistent relativistic discrete-variational X\ensuremath{\alpha} potential. The spectra were interpreted by analyzing the photoelectron-trapping time in different channels inside each atomic sphere. The energy levels of bound states down to the valence levels of the ligands were calculated within the same framework as the continuum in order to set the absolute energy scale. The calculated spectra were compared and found to agree quantitatively with our experimental results.

A first-principles energy-band study of the metallic rutile phase of V${\mathrm{O}}_{2}$ using a general crystal potential and an expansion of the Bloch functions in a linear combination of atomic orbitals is reported. The results are compared with previous work and experimental optical, x-ray absorption and emission, and x-ray photoelectron spectroscopy data. We obtain a large density of states at the Fermi energy; the Fermi surface is found to be determined by the two lowest $d$ bands, at the bottom of the "${t}_{2g}$" manifold which is split by the orthorhombic field; the lowest-band Fermi surface possesses some nesting features corresponding to a nesting vector $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}=\ensuremath{\Gamma}R$. A calculation of the generalized susceptibility in the constant-matrix-element approximation shows the existence of a maximum at the zone boundary $R$. We suggest that the formation of a charge-density wave with wave vector $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}=\ensuremath{\Gamma}R$ accompanied by a periodic lattice distortion is thus possible; the subsequent condensation of phonons at the point $R$ could then explain the crystallographic phase transition observed at $T=339$ K.

The electronic energy bands and wave functions of cubic $3C$-SiC have been calculated in the first-principles Hartree-Fock-Slater model, making use of the discrete variational method. A comparison is made between experimental optical data, calculated indirect transitions, and theoretical results for ${\ensuremath{\epsilon}}_{2}(E)$ and reflectivity. The valence-band density of states is found to be in reasonably good agreement with x-ray emission spectra.

The electronic structure of the ordered Zintl ($B32$) phase of LiAl has been studied in a molecular-cluster model within the framework of the Hartree-Fock-Slater theory. ${\mathrm{Li}}_{5}$${\mathrm{Al}}_{4}$ and ${\mathrm{Al}}_{5}$${\mathrm{Li}}_{4}$ clusters were embedded in a potential field representative of the alloy environment; energy levels and wave functions were obtained by self-consistent iteration. Density-of-states and charge-density results are used to interpret NMR and electrical-conductivity studies. The case of a single Li vacancy was also treated, and discussed in the light of positronannihilation data.

The self-consistent discrete variational method and the local X\ensuremath{\alpha} exchange approximation have been employed to obtain the electronic structure of 15-atom clusters representing Fe metal and TiFe alloys. The dependence of the magnetic moment and isomer shift of iron metal on pressure was investigated. Local moments of Ti dilute impurities in Fe were calculated. The ordered (CsCl-structure) intermetallic compound FeTi, which represents a concentrated-impurity limit of the TiFe alloys, was also studied, and an isomer-shift (IS) value obtained. The results show that the cluster approach yields magnetic moments and IS consistent with experimental magnetization, neutron scattering, and M\"ossbauer data.

Spectroscopic and bonding properties of sulfur, selenium, and tellurium atoms chemisorbed on different crystallographic faces of Ni are studied using self-consistent local-density molecular-cluster models. Adsorbate-substrate interactions lead to significant modification of the Ni $3d$, $4s$, $4p$ conduction bands in addition to forming the $\ensuremath{\sigma}$, $\ensuremath{\pi}$ adatom levels located at \ensuremath{\sim}5 eV below the Fermi energy. Adatom-adatom interactions are treated by ${\mathrm{S}}_{2}$${\mathrm{Ni}}_{9}$ coupled clusters. Adatom bonding to the substrate is seen to be dominated by near-neighbor interactions through $\mathrm{sp}$ hybridization of the low-lying chalcogen $\mathrm{ns}$ level, and by $\ensuremath{\pi}$-bonding interaction of the $\mathrm{np}$ level. The (110) surface appears to have more ionic character than either (001) or (111). The calculated variation of adsorbate levels with adatom height above the metal surface supports low-energy-electron diffraction structural analyses. Partial densities of states derived from cluster orbital atomic populations are used to discuss features of photoelectron and Auger spectra.

Angle resolved oxygen {ital K}-edge x-ray absorption spectroscopy measurements were performed on single crystals of YBa{sub 2}Cu{sub 3}O{sub 6.9}. Measurements were carried out on in situ cleaved crystals at 20 K to minimize the loss of oxygen from the surface region of the samples. It was found that there are two distinct transitions between 526 and 531 eV. The peaks in YBa{sub 2}Cu{sub 3}O{sub 6.9} show strong orientation dependence, as well as energy dispersion of 2 eV. After the sample is heated to room temperature, there is only one peak visible which shows little energy dispersion or orientation dependence. Comparison of the experimental results to a transition-state calculation of the absorption cross section shows good agreement indicating that there are holes of {sigma} symmetry on all Cu-O bonds.

Core-electron-removal energies representing both main-line and satellite features in core-level x-ray photoelectron spectra are calculated for FeCl/sub 2/ with use of a self-consistent embedded-cluster model. Energies are obtained by using the Slater transition-state scheme in both relativistic and nonrelativistic versions of a moment-polarized discrete variational method code. In the model calculations, spectral peaks correspond to energy differences between the ground state and ''well-screened'' or ''poorly-screened'' final states. The study includes considerations of exchange, basis set, effects associated with finite cluster size (e.g., cluster charge), and the correspondence between relaxation and excitation processes. Observed main-line and satellite spectra are successfully calculated, provided that the core hole is treated self-consistently, both with respect to the Hamiltonian and with respect to the choice of variational basis.

The electronic structure and stabilization energy of metal vacancy and vacancy-interstitial clusters are studied in transition-metal monoxides using the first-principles local-density theory. The discrete-variational method in the embedded-cluster scheme is used to obtain both the one-electron properties and cohesive energies. Our calculations predict greater stability for 2:1 (vacancy:interstitial) defect structure over simple vacancies. This is in agreement with experiments for MnO and the heavier 3d compounds, but for TiO and VO lattice vacancies at the metallic or oxygen sites are experimentally predominant. The energy for the single metal vacancy is calculated to be close to the 2:1 defect energy, and so cluster-size effects and computational limitations need to be considered further. For FeO and CoO, however, the 4:1 interstitial complex is found to be more stable than other simple defects, in accord with experiment. The probability of formation of 4:1 clusters and their aggregates in TiO and MnO is explored; the interstitial metal ion tends to be in a trivalent state as previously determined for FeO and CoO.

The self-consistent local-density theory is used in a cluster model to calculate the charge distribution, one-electron energy spectra, and the density of states for pure InP and for H and V substituted at the In site. The pure semiconductor gap is found to be 0.8 eV, consistent with experiments. The hydrogen impurity introduces a trapping center at E/sub v/+0.37 eV in the gap region: a semi-insulating-like behavior. The transition metals are generally found to introduce deep levels in the gap region, but for vanadium we find the gap swept clean of any impurity level, with the last partially occupied state close to the bottom of the conduction band. This is consistent with deep-level transient spectroscopy experiments where no vanadium-related deep levels in the band gap are found down to 4 K.

The electronic energy levels and charge distribution associated with kink (K) and kink-antikink (K-K\ifmmode\bar\else\textasciimacron\fi{}) defects in polyacetylene have been calculated with use of the first-principles local-density-functional theory. Finite-chain models with up to 40 carbon atoms were studied, in a self-consistent-field molecular-orbital approach. For the kink defect on trans-polyacetylene (PA), a half-filled midgap level was found, which corresponds to a neutral soliton having spin (1/2. Its formation energy was found to be 0.51 eV. Two defect levels were found for kink-antikink pair constructed on both the trans- and cis-PA chains. In the trans case, the formation energy is about 1.1 eV, which remains almost constant with increasing separation between K and K\ifmmode\bar\else\textasciimacron\fi{}. The formation energy in the cis case increases monotonically with increasing separation and ranges from 0.3 eV for two intervening C atoms to 1.5 eV for 18 C atoms between K and K\ifmmode\bar\else\textasciimacron\fi{}.

The electronic structure of the high-temperature rutile phase of Nb${\mathrm{O}}_{2}$ is studied by the linearized-augmented-plane-wave method. Potentials constructed by superposition of neutral-atom and ionic-charge densities are used to explore variability of the electronic band structure. A rigid-band scheme is shown to accurately describe optical absorption of the rutile phase of Nb${\mathrm{O}}_{2}$ stabilized by the addition of 20 at.% Ti as measured by Raccah et al. Differences between the band results for rutile Nb${\mathrm{O}}_{2}$ and the optical absorption measurements on the low-temperature body-centered tetragonal phase of Nb${\mathrm{O}}_{2}$ are attributed to band splittings induced by lattice distortion which occur at the phase transition. The static-electron response function $\ensuremath{\chi}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}})$ is calculated in the constant-matrix-elements approximation. In contrast to the case of isoelectronic V${\mathrm{O}}_{2}$, no Fermi-surface nesting features are observed, and $\ensuremath{\chi}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}})$ is found to be structureless in the vicinity of the point $P=(\frac{1}{4}, \frac{1}{4}, \frac{1}{2})$ which has been associated with a possible soft-mode phonon instability responsible for the lattice transformation.

The layered structure dichalcogenide Ti${\mathrm{S}}_{2}$ was studied in thin-film and bulk models, with the use of the self-consistent linearized augmented-plane-wave (LAPW) scheme. Band-structure calculations were performed on the single sandwich (S-Ti-S), the double sandwich ${(\mathrm{S}\ensuremath{-}\mathrm{T}\mathrm{i}\ensuremath{-}\mathrm{S})}_{2}$, and the bulk to reveal the strength and effect of interlayer interactions on the electronic structure. Comparisons are made with existing bulk band models. The fully intercalated LiTi${\mathrm{S}}_{2}$ compound was studied by the same LAPW methods. Changes in the electronic density of states and charge density induced by the alkali intercalate are described.

Local-density theory is used to study the electron charge-density distribution around hydrogen and host palladium metal atoms. Self-consistent calculations using a finite-size molecular-cluster model based on the discrete variational method are reported. Calculations are also done in a simple ''pseudojellium'' model to study the electron response to hydrogen within the framework of the density-functional formalism. Results of this simple approach agree very well with the molecular-cluster model. Partial densities of states obtained in the cluster model are compared with band-structure results and conclusions regarding the importance of the local environment on the electronic structure are drawn. Calculated core-level shifts and charge transfer from metal ions to hydrogen are compared with the results of x-ray--photoelectron spectroscopy experiments in metal hydrides and are discussed in terms of conventional anionic, covalent, and protonic models. The effect of zero-point vibration on the electron charge and spin-density distribution is studied by repeating the above calculations for several displaced configurations of hydrogen inside the cluster. The results are used to interpret the isotope effect on the electron distribution around proton and deuteron.

Self-consistent embedded cluster models are developed to describe the electronic structure of CoGa, and of isolated transition-metal impurities (Ti, V, Cr) in CoGa. First-principles local-density theory is used in a variational linear combination of atomic orbitals framework to obtain effective charge and spin configurations of host and impurity atoms. 9, 15, and 27 atom clusters are considered; charge configurations are somewhat sensitive to cluster size, and spurious spin polarization of peripheral Co atoms is observed in the smaller clusters. The large-cluster results are consistent with experimental interpretations.

The x-ray $K$-emission spectrum and the soft-x-ray absorption spectrum are calculated for diamond and compared with experiment. The influence of transition matrix elements, calculated in a one-electron approximation, is discussed. Binding-energy corrections on the core and valence states are estimated and used to explain observed shifts between band transition energies and experimental data. Approximate methods are used to study localized valence relaxation effects on the transition matrix elements in emission.

The systematics of the electronic structure of $\mathrm{La}B{\mathrm{O}}_{3}$ perovskite oxides, where the $B$ element scans the $3d$ transition-metal series from Ti to Co, are examined. X-ray photoelectron spectra of valence bands and shallow core states are presented and compared with theoretical molecular cluster and free-ion multiplet models. Self-consistent ionic configurations are obtained from the embedded cluster calculations, which differ from the assumptions of crystal-field theory due to metal-oxygen covalency. The prospect for more rigorous many-electron models is discussed.

Electronic structure calculations were performed for clusters representing the Chevrel-phase ${\mathrm{SnMo}}_{6}$${\mathrm{S}}_{8}$, with and without oxygen doping. In order to obtain the local structure around the Sn atom, extended x-ray-absorption fine-structure (EXAFS) measurements were made with synchro- tron radiation. The interatomic distances obtained experimentally were used in the calculations. The effect of oxygen doping on the M\"ossbauer isomer shift and quadrupole splitting values of $^{119}\mathrm{Sn}$ was investigated theoretically and compared with reported experimental values. The effect of oxygen substitution on the density of states at the Fermi energy of the [${\mathrm{Mo}}_{6}$${\mathrm{S}}_{8}$${]}^{2\mathrm{\ensuremath{-}}}$ cluster was also studied. The results suggest that oxygen doping does not alter significantly the electronic structure of ${\mathrm{SnMo}}_{6}$${\mathrm{S}}_{8}$.

Total energies as a function of carbon-metal-layer separation and carbon-metal perpendicular stretch frequencies are calculated for small clusters configured to be analogs for the chemisorption system CO on Cu/Ru(0001). The clusters (with 7--10 metal atoms) include analogs of CO/Cu(111) and CO/Ru(0001) with on-top and threefold binding sites. The latter are more stable in both cases. Mixed-metal clusters are also studied (i) with CO bound to Cu in proximity to Ru and (ii) with CO bound to Ru in proximity to Cu. The former, case (i), show enhanced CO binding with respect to CO on pure Cu, and the latter, case (ii), show weakened CO binding with respect to CO on pure Ru. Possible precursor intermediate states are described for commensurate and incommensurate epitaxy. Bonding to Ru sublayers through a Cu adlayer is also described. The results are discussed in conjunction with previous experimental findings, and anticipate future experimental vibrational characterizations.

Substitutional transition-metal impurities occupying either Ni or Al sites in the intermetallic compound ${\ensuremath{\beta}}^{\ensuremath{'}}$-NiAl with CsCl structure have been treated by a self-consistent molecular-orbital method. The electronic energy levels and charge and spin densities of $M{X}_{8}$ clusters representing a central atom and its nearest neighbors and $M{X}_{8}{Y}_{6}$ clusters representing central, nearest-neighbor and second-nearest-neighbor atoms have been calculated in the Hartree-Fock-Slater spin-polarized model, using a numerical discrete variational method. Interaction of the impurity cluster with the crystal environment is represented by a pseudopotential derived from cluster calculations on the pure compound. Results for Fe, Ni, and Co impurities in ${\ensuremath{\beta}}^{\ensuremath{'}}$-NiAl are discussed in connection with experimental resistivity, M\"ossbauer isomer shift, and NMR data.