Showing papers on "Electronic structure published in 1977"

Methods of Electronic Structure Theory

Abstract: 1. Gaussian Basis Sets for Molecular Calculations.- 2. The Floating Spherical Gaussian Orbital Method.- 3. The Multiconfiguration Self-Consistent Field Method.- 4. The Self-Consistent Field Equations for Generalized Valence Bond and Open-Shell Hartree-Fock Wave Functions.- 5. Pair Correlation Theories.- 6. The Method of Configuration Interaction.- 7. The Direct Configuration Interaction Method from Molecular Integrals.- 8. A New Method for Determining Configuration Interaction Wave Functions for the Electronic States of Atoms and Molecules: The Vector Method.- 9. The Equations of Motion Method: An Approach to the Dynamical Properties of Atoms and Molecules.- 10. POLYATOM: A General Computer Program for Ab Initio Calculations.- 11. Configuration Expansion by Means of Pseudonatural Orbitals.- Author Index.

The spectrum of molecular nitrogen

Abstract: This is a critical review and compilation of the observed and predicted spectroscopic data on the molecule N2 and its ions N2 −, N2 +, N2 2+, and the molecule N3. Each electronic band system is discussed in detail, and tables of band origins and heads are given. In addition to the gas phase electronic, electron and Raman spectra, there are also examined the spectra of condensed molecular nitrogen as well as the pressure‐ and field‐induced infrared and microwave absorption. Dissociation energy of N2, predissociations, and perturbations are discussed. Potential energy curves are given, as well as radiative lifetimes, f‐values, and Franck‐Condon integrals. Molecular constants are listed for the known electronic states. Electronic structure and theoretical calculations are reviewed.

Gaussian Basis Sets for Molecular Calculations

Abstract: In the following chapters the electronic structure of molecules will be discussed and the techniques of electronic structure calculations presented. Without exception the molecular electronic wave functions will be expanded in some convenient, but physically motivated, set of one-electron functions. Since the computational effort strongly depends on the number of expansion functions (see, e.g., the following chapters), the set of functions must be limited as far as possible without adversely affecting the accuracy of the wave functions. This chapter will discuss the choice of such functions for molecular calculations.

Chemistry: The Central Science

Abstract: 1. Introduction: Matter and Measurement. 2. Atoms, Molecules, and Ions. 3. Stoichiometry: Calculations with Chemical Formulas and Equations. 4. Aqueous Reactions and Solution Stoichiometry. 5. Thermochemistry. 6. Electronic Structure of Atoms. 7. Periodic Properties of the Elements. 8. Basic Concepts of Chemical Bonding. 9. Molecular Geometry and Bonding Theories. 10. Gases. 11. Intermolecular Forces, Liquids, and Solids. 12. Modern Materials. 13. Properties of Solutions. 14. Chemical Kinetics. 15. Chemical Equilibrium. 16. Acid-Base Equilibria. 17. Additional Aspects of Equilibria. 18. Chemistry of the Environment. 19. Chemical Thermodynamics. 20. Electrochemistry. 21. Nuclear Chemistry. 22. Chemistry of the Nonmetals. 23. Metals and Metallurgy. 24. Chemistry of Coordination Compounds. 25. The Chemistry of Life: Organic and Biological Chemistry.

The Method of Configuration Interaction

Abstract: As the scope of quantum chemistry broadened from the consideration of stable molecules near equilibrium to encompass potential curves and surfaces, transition states, radicals, ions, and excited states, the shortcomings of the Hartree— Fock (HF) approximation for the description of the electronic structure of molecular systems became increasingly evident. The energy error of the restricted HF wave function, i.e., the difference between the HF limit energy (which is the limit approached by restricted self-consistent field calculations as the basis set approaches completeness) and the exact solution of the nonrelativistic Schrodinger equation, has been termed the correlation energy.(1) It reflects the fact that the HF Hamiltonian contains the average, rather than instantaneous, interelectron potential, and thus neglects the correlation between the motions of the electrons.

Electron states in α-quartz: A self-consistent pseudopotential calculation

Abstract: Self-consistent pseudopotentials are used to investigate the electronic structure of $\ensuremath{\alpha}$-quartz. We present calculations for the band structure, electronic density of states, optical response functions, pseudocharge densities, and x-ray emission spectra. We find excellent agreement between theory and experiment with respect to photoemission and uv absorption data. The chemical bonding present in $\ensuremath{\alpha}$-quartz as determined from pseudocharge density contour maps is consistent with other theoretical calculations. The theoretical x-ray emission spectra, as obtained from an orthogonalized-plane-wave scheme, are compared with experimental data. The calculated silicon and oxygen $K$ spectra agree very well with experiment; however, the $\mathrm{Si} {L}_{2,3}$ spectrum exhibits substantial disagreement with the data. An explanation is proposed based upon the formation of amorphous elemental Si in Si${\mathrm{O}}_{2}$ during electron irradiation.

Semiempirical Methods of Electronic Structure Calculation

Abstract: 1. Ground-State Potential Surfaces and Thermochemistry.- 1. Introduction.- 2. Macroscopic Properties from Molecular Calculations.- 2.1. A Scheme for Thermodynamic Parameters.- 2.2. The Need for Geometry Calculations.- 2.3. Statistical Thermodynamic Formalism.- 2.4. Activation Parameters.- 2.5. The Zero-Point Vibrational Correction.- 2.6. The Partition Function.- 3. Semiempirical Molecular Orbital Theory for Closed Shells.- 3.1. The Nature of Semiempirical Theory.- 3.2. Parametrization.- 3.3. Strengths and Limitations for Potential Surface Calculations.- 4. Exploring Potential Energy Surfaces.- 4.1. The Size and Shape of Potential Surfaces.- 4.2. Geometry Optimization.- 4.3. Force Constants.- 5. Selected Results and Comparisons.- 5.1. Introduction.- 5.2. Molecular Geometries.- 5.3. Energies of Equilibrium States.- 5.4. Activation Parameters.- 5.5. Vibrational Frequencies.- 6. Conclusions and an Opinion.- References.- 2. Electronic Excited States of Organic Molecules.- 1. Introduction.- 2. The Hamiltonian Operator.- 3. The Zeroth-Order Approximation.- 4. The Electronic Wave Function.- 4.1. The All-Valence-Electron Approximation.- 4.2. The SCF Procedure.- 4.3. The Trial Functions.- 4.4. The ZDO Approximation.- 4.5. The Semiempirical Approximations.- 4.6. Comparison of Various Methods.- 5. The Interaction of Matter and Electromagnetic Fields.- 5.1. Transition Moments.- 5.2. Photoelectron Cross Sections.- 6. Spin-Orbit and Spin-Spin Coupling.- 6.1. Spin-Orbit Coupling.- 6.2. Spin-Spin Coupling.- 7. Vibrationally Induced Transitions.- 7.1. Herzberg-Teller Theory.- 7.2. Born-Oppenheimer Breakdown Theory.- 8. Application of ZDO Methods.- 8.1. Simple Organic Compounds.- 8.2. Inorganic Compounds.- 8.3. Interacting Nonplanar 7r-Electron Systems.- 8.4. TripletStates.- 8.5. Free Radicals and Doublet States Photoelectron Spectra.- 8.6. Rydberg Transitions.- 8.7. Treatment of d Orbitals.- 8.8. Geometry of Excited States.- 8.9. Spin-Orbit, Spin-Spin, and Vibronic Coupling.- 8.10. Ionization Potentials.- 8.11. Dipole Moments and Polarizabilities.- 8.12. Miscellaneous Studies.- 9. Conclusions.- References.- 3. Photochemistry Josef Michl.- 1. Introduction.- 2. Photochemical Processes.- 3. Semiempirical Methods.- 3.1. Model Hamiltonians.- 3.2. Solving the Models.- 4. Examples of Application.- 4.1. Phototautomerism.- 4.2. Electrocyclic Reactions.- 5. Summary and Outlook.- References.- 4. Approximate Methods for the Electronic Structures of Inorganic Complexes.- 1. Inorganic Complexes Contrasted to Organic Molecules.- 2. TheOrbitals.- 3. The Ligand Field and the Crystal Field Methods.- 4. Koopmans' Theorem.- 5. Spin-Orbit Coupling.- 6. NonempiricalCNDO and INDO Methods.- 7. Semiempirical CNDO and INDO Methods.- 8. The Excited States.- 9. The Crystal Field Theory.- 10. Extended Huckel Theory. Angular Overlap Model.- 11. An Example, Ni(CN)4~. Conclusions.- References.- 5. Approximate Molecular Orbital Theory of Nuclear and Electron Magnetic Resonance Parameters.- 1. Introduction.- 2. Magnetic Resonance Parameters.- 3. Molecular Quantum Mechanics.- 3.1. Molecular Orbital Theory.- 3.2. Approximate Molecular Orbital Theory.- 3.3. Perturbation Theory.- 4. NMR Shielding Constants and Chemical Shifts.- 4.1. Quantum Mechanical Development of ?N.- 4.2. Calculation of Shielding Constants.- 5. NMR Nuclear Spin Coupling Constants.- 5.1. Quantum Mechanical Development of KMN.- 5.2. The Fermi Contact Term.- 5.3. The Orbital and Dipolar Terms.- 5.4. Calculations of JMN.- 6. ESRg-Tensors.- 7. ESR Electron-Nuclear Hyperfine Tensors.- 7.1. Quantum Mechanical Development of TN.- 7.2. Isotropic Hyperfine Coupling.- 7.3. Calculations of Isotropic Hyperfine Constants.- 7.4. Anisotropic Hyperfine Coupling.- 7.5. Calculations of Anisotropic Hyperfine Constants.- References.- 6. The Molecular Cluster Approach to Some Solid-State Problems.- 1. Introduction.- 1.1. Perfect Crystalline Solids and the Bloch Theorem.- 1.2. Imperfect Solids and the Breakdown of B loch's Theorem.- 2. Solid-State Theory Approaches to Surface Problems.- 2.1. The Perfect Surface.- 2.2. Surf ace-Adsorbate Interactions.- 3. Molecular Cluster Approach to Surface Problems.- 3.1. Nonmetals.- 3.2. MetalClusters.- 3.3. Metals and Adsorbates.- 4. Summary.- References.- 7. Electron Scattering Donald G. Truhlar.- 1. Introduction.- 2. Explicit Inclusion of Electronic Excitations.- 2.1. Expansions Including Free Waves.- 2.2. L2 Expansions.- 3. Neglect of Electronic Excitation Except for Final State.- 3.1. Strong-Coupling, Static-Exchange, and Distorted-Wave Approximations.- 3.2. High-Energy Approximations.- 4. Inclusion of Effect of Omitted Electronic States by Approximate Polarization Potentials.- References.- Author Index.- Molecule Index.

Geometries and energies of the excited states of O3 from ab initio potential energy surfaces

Abstract: The geometries and relative energies of the nine lowest states of the ozone molecule have been determined in C2v symmetry from ab initio configuration interaction calculations in a [3s2p1d] contracted Gaussian basis Calculations were carried out over a two‐dimensional grid of points in C2v symmetry to locate the optimum geometrical parameters R and ϑ for each state For the ground 1A1 state the calculated properties (with experimental values in parentheses) are as follows: Re=1299 A (1271 A), ϑe=1160° (1168°), ω1=1235 cm−1 (1110 cm−1) and ω2=707 cm−1 (705 cm−1) Of the excited states only the lowest 3B2 state is found to have an adiabatic excitation energy (092 eV) less than the dissociation energy (De=113 eV) and hence to be a likely bound species The 1B2 state responsible for the strong absorption in the Hartley band (47–58 eV) is stabilized by asymmetric distortions away from its equilibrium C2v geometry (Re=1405 A, ϑe=108°) suggesting unequal bond lengths for this state or else purely disso

Electronic structure of aluminum and aluminum-noble-metal alloys studied by soft-x-ray and x-ray photoelectron spectroscopies

Abstract: X-ray photoelectron spectra (XPS) and $\mathrm{Al} K$ and $\mathrm{Al} {L}_{2,3}$ soft-x-ray spectra (SXS) of valence bands for aluminum-noble-metal alloys are rationalized on the basis that SXS spectra are dominated by dipole selection rules while XPS spectra chiefly reflect the noble-metal $d$ bands. We find, in agreement with theory, that the main peaks in the $\mathrm{Al} 3s$ partial densities of states are at higher binding energy than those for the noble metal $d$ states. Peaks in the $\mathrm{Al} 3p$ partial densities of states approximately coincide with the higher-energy peaks in the partial densities of noble-metal $d$ states. The remarkable large chemical shifts of core levels in these alloys are noted and the $\mathrm{Au} {N}_{6,7}$ SXS emission from aluminum-noble-metal alloys also discussed. The Auger spectra from Al-Ag and Al-Cu alloys provide strong additional evidence that single-atom effects dominate in the Auger processes for these alloys.

Structure and thermodynamics of liquid metals and alloys

Abstract: Ab initio calculations of the structure and the thermodynamic properties of simple liquid metals and alloys are presented. The basic ingredients of the theory are as follows: (a) An orthogonalized-plane-wave-based first-principles pseudopotential is used to describe the interatomic forces. The pseudopotential is optimized specifically for binary systems. The $X\ensuremath{\alpha}$ method is used to construct the electron-ion potential. (b) A system of hard spheres is used as a reference system for describing the liquid structure. Effective hard-sphere diameters are determined by a variational method based on the Gibbs-Bogolyubov inequality. The method of this paper yields encouraging results for the structure factors, the excess volume, and the enthalpy and entropy of formation for alloys with a nearly random distribution of the components. Difficulties arise where nonrandomness has to be expected. The theory provides a basis for a microscopic understanding of the thermodynamics of alloys of simple metals in terms of their electronic structure.

Self-consistent numerical-basis-set linear-combination-of-atomic-orbitals investigation of the electronic structure and properties of TiS2

Abstract: A fully-self-consistent numerical-basis-set linear-combination-of-atomic-orbitals calculation of the electronic structure of Ti${\mathrm{S}}_{2}$ is reported using the method described previously. The calculated band structure differs considerably from those previously obtained by non-self-consistent muffin-tin models. Comparison with experiment shows that the calculated optical properties for energies below 16 eV and the various characteristics of the valence and conduction bands agree very well with optical-absorption and electron-energy-loss data as well as with photoemission, x-ray absorption, and appearance-potential spectra. A small indirect gap (0.2-0.3 eV) occurs at the points $M$ and $L$ in the Brillouin zone with a larger direct gap (0.8 eV) at $\ensuremath{\Gamma}$. We suggest that the characteristic semi-metallic large $g$ value observed experimentally originates from a near coincidence of the band gap with the enhanced spin-orbit splitting which is consistent with the soft-x-ray data and our band model. The bonding mechanism in Ti${\mathrm{S}}_{2}$ is discussed in detail; it is shown by a direct calculation of the self-consistent charge density and the transverse effective charge that the system is predominantly covalent with small static ionic character and large dynamic ionicity. In contrast with muffin-tin $X\ensuremath{\alpha}$ models, the bonding is found to be largely due to Ti $4s4p$ to S $3p$ bonds and a much weaker Ti $3d$ to S $3p$ bond. The effects of muffin-tin approximation and self-consistency are discussed in detail. Extrapolation of these results to the case of Ti${\mathrm{Se}}_{2}$ is made and the possible origin of its charge-density wave is discussed.

Ground- and excited-state properties of LiF in the local-density formalism

Abstract: The band structure, charge density, x-ray scattering factor (and their behavior under pressure), equilibrium lattice constant, and cohesive energy of the prototype ionic solid LiF were determined using our recently developed self-consistent numerical basis set (non-muffin-tin) linear-combination-of-atomic-orbitals method, within the local-density formalism (LDF). The details of the bonding and the effects of exchange and correlation on the electronic structure are discussed with reference to the conventional picture of ionic bonding. Remarkably good agreement is found with the observed data for the ground-state properties of the system. Contrary to the results of previous band studies, the conventional band-structure approach to excitation energies (i.e., identifying them with the band eigenvalue differences) is found to fail completely in accounting for the observed data in the entire x-ray and optical spectral region when fully self-consistent solutions of the LDF one-particle equation with no further approximation to the crystal potential are obtained. It is found that in the presence of some spatial localization of the initial or final crystal states, the spurious self-interaction terms, as well as the polarization and orbital relaxation self-energy effects are of a similar order of magnitude as the Koopmans'-like interband terms. In order to treat these effects within the LDF self-consistently, we describe the excitation processes as transitions involving point-defect-like states in the solid calculated by a supercell method in which the excitation energies are determined as total-energy differences between (separately calculated) excited- and ground-state configurations. The excited state is represented as a superlattice of locally excited sites using large (8-and 16-atom) unit cells, each containing a single excited site. We find, in the self-consistency limit, that a small but finite degree of spatial localization of the excited states exists even for valence excitations, inducing thereby self-interaction as well as self-energy relaxation and polarization effects. The LDF model is found to account very well for both interband and exciton transitions over the entire spectral region (12-695 eV) and to yield definite predictions regarding the exciton bandwidths and series limits.

Electronic Structure of Rutile Oxides TiO2, RuO2 and IrO2 Studied by X-ray Photoelectron Spectroscopy

Abstract: Valence band X-ray photoelectron spectra of the rutile oxides TiO2, RuO2, and IrO2 show a distinct evolution as a function of the increasing number of metal d-electrons. The observations are found to be in excellent agreement with recent theoretical models. Intensity analysis and photo-ionization cross sections are used to confirm the nature of the band responsible for conducting properties. The result is in accordance with a crystal field model proposed earlier. The deduced number of free electrons in the valence band is used to correlate the asymmetry recorded for the core level lines.

Surface and interface states on GaAs(110): Effects of atomic and electronic rearrangements

Abstract: Research during the last year has led to a better understanding of the electronic and atomic structure of the (110) surfaces of III–V semiconductors. In this paper we will briefly review these new developments as well as point out areas where agreement has been found between various experimental results presented in the literature. It is now generally agreed that there are no intrinsic surface states in the band gap on GaAs and the smaller band‐gap materials (e.g., GaSb, InAs, and GaSb) and that Schottky barrier pinning must be due to states produced when the metal adlayer is applied. Particular attention is focused in this paper on the large surface rearrangement which takes place on the (110) GaAs surface and effects of the strain which may be produced in joining this rearranged surface layer to the rest of GaAs crystal. It is pointed out that this may lead to variations in the surface rearrangement which can produce variations in the valence electronic structure at the surface. Such variations are shown in experimental energy distribution curves obtained by the photoemission technique which samples principally the last two molecular layers. It is further shown that surprisingly small amounts of chemisorbed oxygen can produce first‐order effects in the valence‐band electronic structure. On all GaAs (110) surfaces studied, a phaselike transformation was observed with a few hundredths of a monolayer coverage of chemisorbed oxygen. Near this coverage, the Ga 3d exciton structure disappears and the oxygen uptake increases significantly. On certain samples, first‐order changes in the valence‐band electronic structure were observed at a coverage of a hundredth of a monolayer or lower. These transformations are discussed in terms of the electronic and atomic configurations at the surface. Experimental data showing As and Ga 3d chemical shifts for oxidation as well as chemisorption are also presented and used to point out difficulties to be expected in passivating practical surfaces. In particular, the effect of mixed As and Ga oxides, the desirability of bonding passivating layers to the GaAs through As bonds, and the effect of strain‐induced interface states are discussed.

Stereoelectronic properties of photosynthetic and related systems. 1. Ab initio quantum mechanical ground state characterization of free base porphine, chlorin, and ethyl pheophorbide a

Abstract: Ab initio SCF calculations on the ground state of free base porphine, chlorin, and ethyl pheophorbide a have been carried out using the molecular fragment procedure. Molecular orbital energies and ordering, and the correlations of specific orbitals among the molecules studied, have been examined in detail. Ionization potentials have been estimated, and the first ionization potentials are 6.8, 6.4, and 6.4 eV for porphine, chlorin, and ethyl pheophorbide a, respectively. The calculations show the expected approximate separation of the HOMO, HOMO--1, LUMO, and LUMO + 1 from the remainder of the MO manifold in keeping with the ''four orbital'' model, and isodensity contour plots of occupied and unoccupied molecular orbitals indicate a striking similarity in the ''shapes'' of these orbitals in all three molecules. Charges and bond orders have been examined. In porphine and chlorin, the bonding picture includes an extended ..pi.. system whose path of conjugation involves the atoms of the interior of the macrocycle, including the nitrogens and methine carbons. Also, relatively localized ..pi.. bonds are found between the exterior carbon atoms of the pyrrole moieties. In ethyl pheophorbide a, the ..pi.. bonds of the keto carbonyl of ring V and the vinyl group of ring I aremore » mostly localized, but the path of conjugation within the macrocycle is somewhat less clear. Finally, molecular electrostatic isopotential maps have been constructed and an analysis of the long-range electrostatic field and its relationship to intermolecular interactions is discussed.« less

A theory of Fano effects in the electronic structure of d band metals and alloys

Abstract: The electronic structure of d band metals and alloys is studied by using the concept of the Fano effect which is commonly used in solid state optics. A simple theory based on the Anderson model is presented to explain the asymmetry in the local density of states (DOS) of s and p symmetry in a d band metal. A prescription is given for calculating the parameters in the Anderson model from first principles and some simple example calculations are carried out to illustrate AR. An antiresonance (AR) dip position in a local DOS at a given site is quite insensitive to a change in the potential at the site, while the d band at the surrounding sites is shifted due to the potential change. It is pointed out that this shift of the d band results in AR in a generalised phase shift Delta NL, that is, a change in the number of occupied states due to the potential change. The significance of AR in Delta NL in several phenomena is discussed.

Thermally induced breakdown of the direct-transition model in copper

Abstract: A strong temperature dependence has been observed for the first time in angle-resolved photoemission (ARP) spectra of the valence band of a crystalline solid. This spectral behavior confirms predictions of a model suggested by Shevchik. A controversial point in the interpretation of ARP spectra at x-ray energies is resolved by this model. Moreover, it dictates the choice of photon energy and sample temperature for future ARP studies of valence-band electronic structure.