A new method of calculating the total energy and other ground-state properties of solids which employs nonlocal pseudopotentials in conjunction with the variational quantum Monte Carlo approach is presented. Valence electron correlations are treated using the exact interaction with a correlated many-electron wavefunction of the Jastrow-Slater form. The use of pseudopotentials for the electron-ion interaction removes from the problem the large fluctuations of electron energies in the core region which occur in quantum Monte Carlo all-electron schemes. We discuss calculation of the cohesive energy and structural properties of diamond and graphite and the ionization energy and electron affinity of atoms using the present approach. The results are in excellent agreement with experiment.

A self-consistent orbital-based scheme is applied to metallic systems and is found to be efficient for these materials. The technique uses the local density approximation in the pseudopotential framework, but replaces the planewave basis by a basis pseudoatomic orbitals constructed directly from the pseudoatoms. Free electron-like wavefunction components are handled by orthogonalizing the orbital basis to a few low energy planewaves as needed for good eigenvalue and total energy convergence. The method is fast and versatile enough to be used for a variety of problems. Applications to bulk bandstructures, total energies, and forces in Al and Nb are presented.

We propose a simple electronic model for the cohesive energy of metals which explicitely considers eigenfunctions and eigenvectors in a single-particle LCAO approach. From them, forces are obtained to perform molecular dinamics or static relaxation. As a first application we compare the model predictions to full many-body ab-initio and Embedded Atom Model calculations of structure and energetics of small metallic clusters. Adventages and limitations of this approach as well as size restrictions are discussed.

A new, approximate method has been developed for computing total energies and forces for a variety of applications including molecular dynamics simulations of covalent materials. The method is tight-binding-like and is based on the local density approximation within the pseudopotential scheme. Slightly excited pseudo-atomic-orbitals are used, and the tight-binding Hamiltonian matrix is obtained in real space. The method is used to find the total energies for five crystalline phases of Si and the Si 2 molecule. Excellent agreement is found with experiment. A molecular dynamics simulated annealing study has been performed on the Si 3 molecule to determine the ground state configuration.

The Embedded Atom Method (EAM), a modem theory of metallic bonding, has been developed to provide a simple but accurate method of evaluating the energy and forces for an arbitrary arrangement of atoms. The relationships between the EAM and the underlying electron density theory will be discussed. Specific examples of EAM calculations of surface reconstruction for (110) fcc materials will be predicted and compared to experiment. The examples will include temperature effects in gold. The results of molecular dynamics calculations of the mechanical properties of nickel also will be presented. Topics to be discussed include dislocation mobility and dislocation emission from a stressed crack in nickel. The dislocation calculations will be related to continuum modelling.

We briefly discuss some of the advantages and limitations of using embedded atom interatomic potentials for simulating the static configuration and dynamics of lattice defects. In metals, the embedded atom potentials provide a physically more realistic approximation than simple pair interaction potentials without a significant increase in computer time needed for defect simulation studies. However, in some cases, n-body shear forces, i.e bond angle interatomic forces may be needed for fitting experimental results related to defect configuration. One such example is the elastic neutron scattering data from N interstitials in Nb [1]. Also, such bond angle forces must be included in a realistic model of atomic interactions in metals, expecially in highly anisotropic bee transition metals. Extending the concept of the embedded atom method, we propose a new form for the interatomic potential in metals which includes bond angle forces. General expressions for the elastic constants in bee and fee structures are deduced.

An Embedded Atom Method Model for graphite has been derived based on a short-range Morse two-body potential and an electron density function with both radial and angular terms. This part of the model involves interaction only within a hexagonal plane, and the interaction between planes is approximated by a Buckingham potential. The model is stable with respect to fcc, bcc, and diamond structures. The effective two-body potential is very small, indicating that defect properties are dominated by the noncentral many-body interactions.

The application of Tersoff-type potentials are illustrated for examples from three distinct classes of chemical systems: a covalently-bonded solid (carbon), a gas-phase few-body reactive system (H3), and a molecular solid (oxygen). For carbon, the potential energy expression yields stable planar graphite layers as well as a stable diamond phase, both with correct densities and binding energies. The potential also predicts the π-bonded chain structure to be the energetically-preferred reconstruction on the {111} surface of diamond, in agreement with ab initio studies. For hydrogen the potential provides a quantitative description of accurate ab initio energies. The oxygen potential describes both the O2 and O3 molecules, and qualitatively describes intramolecular bonding in the condensed phase.

The use of interatomic potentials in the simulation of ionocovalent solids is discussed. Some of the problems that may arise in the application of atomistic simulations to these solids are highlighted and some criteria for the validation of interatomic potentials for this class of solids are presented.

A new algorithm for generating fully-coordinated hydrogenated amorphous silicon models with periodic boundary conditions is presented. The hydrogen is incorporated into an a-Si matrix by a bond-switching process similar to that proposed by Wooten, Winer, and Weaire, making sure that four-fold coordination is preserved and that no rings with less than 5 members are created. After each addition of hydrogen, the structure is fully relaxed. The models so obtained, to be used as input to molecular dynamics simulations, are found to be in good agreement with experiment. A model with 12 at.% H is discussed in detail.

A self energy approach directly taking into account the many-body nature of the electron-electron interaction is described which gives an excellent account of the quasiparticle band energies in semiconductors and insulators. The self energy approach provides a crucial link between structural models and spectroscopic probes of materials systems. Applications to bulk semiconductors, semiconductor surfaces and short period superlattices are described. A model for the screened Coulomb interaction can reduce the amount of computation required. Applicability of bulk self energy results to more complex systems, e.g. surfaces, is discussed.

In a recent theoretical study of the Pb center at the Si(lll)/SiO2 interface, 1,2 we pointed out the importance of spin-polarization effects for the accurate theoretical description of ESR hyperfine interactions. The small superhyperfine tensor in the Pb center, for example, could not have been definitely assigned to a subset of the second neighbors of the defect atom rather than its nearest neighbors without an explicit consideration of spin polarization effects.

We investigate the concentration and short-range order dependence of the zero-temperature resistivity and thermopower for substitutionally disordered alloys from a first-principles approach. The alloy disorder is simulated by calculating the electronic structure of a large supercell (typically 200–250 atoms) with periodic boundary conditions. For the strong-scattering alloys we consider, the electron mean-free path is much less than the supercell dimension, causing artificial effects of periodicity to be negligible. In spite of strong scattering, there is no evidence for localized states near EF. The resistivity and thermopower are averaged over several configurations resulting in statistical error bounds of approximately ±10%. The concentration-dependent resistivity of substitutional V1−xAlx alloys agree well with Korringa-Kohn-Rostoker coherent potential approximation (KKR CPA) calculations. This confirms the accuracy of KKR CPA theory.

The constrained density functional approach is used to calculate the energy surface as a function of local charge fluctuations in La2CuO4. This energy surface is then mapped onto a self consistent mean field solution of the Hubbard model which allows extraction of the Coulomb interaction parameters when combined with oneelectron parameters derived from band structure results. The present calculations indicate that La2CuO4 is intermediate between the extreme spin or charge fluctuation regimes. This severly restricts the range of parameter space for theories of quasiparticles, optical excitations and possible pairing mechanism based on the extended Hubbard model.

We present a highly accurate tight-binding parametrization of the LAPW band structure of the high-temperature superconductor YBa2Cu3O7, discuss the methodology used in obtaining this fit, and its potential application to a Tight-Binding Coherent-Potential Approximation (TB-CPA) calculation of the effects of oxygen vacancies on the electronic structure.

An accurate tight-binding parametrization of the electronic structure of La2CuO4 is used to investigate the effects of spin polarization of the Cu ion on the band structure and magnetic moment in the antiferromagnetic state. It is found that when an exchange splitting on the d(x2-y2) orbitals is imposed, a gap in the spectrum is obtained, whereas no gap is found if the splitting is imposed on all of the d orbitals or even on both d(x2 -y2) and d(z2) orbitals. This result suggests large anisotropy of the exchange field on the Cu ion.

The self-consistent Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) method is applied to determine the changes in the Cr electronic structure due to addition of up to 30% Ru. The results provide an interpretation of the coexisting antiferromagnetism and superconductivity in terms of specific pieces of Fermi surface. The strong nesting which occurs for the Cr Fermi surface survives the addition of at least 20% Ru, in agreement with the observed persistence of antiferromagnetism into this region.

A method for performing ab-initio investigations of the equilibrium properties of real material solids at finite temperature is discussed. Examples involving surface and bulk structural transitions are presented.

The lattice deformation of the BCC-FCC martensitic transformation in iron can be described as a continuous change of the c/a parameter of the body-centered tetragonal (BCT) lattice from c/a=1 (BCC) to c/a=√2 (FCC). Along this deformation path, the total energy (as a function of volume), the enthalpy (as a function of pressure), the pressure-volume relations--both for nonmagnetic (NM) and ferromagnetic (FM) states--were calculated using the ab initio LMTO method. The ground-state magnetic properties: ferromagnetic contributions to the total energy and magnetic moments, were found by making use of the Stoner theory of itinerant ferromagnetism, rather than spin-polarized calculations. This circumvents the difficulties of using the traditional local spin-density approximation which failes to describe correctly the energetics of iron phases. The Stoner exchange parameter, I, was calculated from the linear response theory for each c/a as a function of volume. Then a constant enhancement factor, β, was introduced and the new Stoner parameter, βl, as used in all the calculations. The factor β was found by fitting the equilibrium atomic volume of the FM BCC phase to its experimental value. No other adjustments of any quantities were performed. The calculations revealed a somewhat unusual behavior of enthalpy along the deformation path. Instead of a double-well curve with a barrier maximum somewhere at 1 < c/a < √2, the enthalpy of the NM phase exhibits a monotonic decrease with c/a, the BCC modification being unstable with respect to the shear deformation. Moreover, up to a certain c/a (depending on pressure), the nonmagnetic BCT phase is also unstable with respect to spontaneous magnetization. Ferromagnetism stabilizes the BCT phases. However, the FM FCC phase is unstable with respect to shear deformation. The enthalpy curve along the deformation path then has a cusp corresponding to a first-order phase transition between ferromagnetic and nonmagnetic states accompanied by an appreciable volume discontinuity. The bulk modulus, the magnetic moments and the BCC-FCC enthalpy differences are in good agreement with the available experimental data.

The electronic structure of Al3V vs its two different crystal structures (DO22 and Ll2) were investigated using local density total energy approach. The calculated results of the total energy showed that in Al3V the tetragonal DO22 phase is energetically favored as compared to the cubic Ll2 phase, the total energy in the former case is about 60 mRy/F.U. lower than that in the later case. The calculated lattice constant (a=3.72 Å, c=8.20 Å) is in fairly good agreement with experiment (a=3.778 Å, c=8.326 Å),and the bulk modulus (1.3 Mbar) is comparable with the experimental Young modulus (150 GPa) for Al3Ti. Furthermore, it is interesting to note that the density of states at EF in the tetragonal DO22 phase (0.14 states/eV-F.U.) is about one order magnitude smaller than that in the Ll2 phase (2.89 states/eV-F.U.). The electronic structure of Al3V seems to be fairly satisfactory in explaining its phase stability.