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We investigate the structural properties, formation energies, and electronic structure of oxygen impurities in GaAs using first-principles total-energy calculations. Five charge states of oxygen occupying an arsenic site (OAs) and various interstitial sites (O i) were considered. For the OAs defect in negative charge states we find off-center configurations withC2v symmetry as reported experimentally. Our results for the formation energies reveal a negative-U behavior for the OAs defect, in which the paramagnetic 22 charge states is never stable. For the Oi defect, we find three equilibrium configurations for the O atom, which are present in all the charge states investigated. The stable configuration for the neutral defect shows the O atom between an As-Ga pair forming the As-O-Ga structure. However, for the negative charge states, the stable configuration shows the O atom exactly at the tetrahedral interstitial site, bonding with four gallium first neighbors. Further, we find that the 12 charge state is never stable, suggesting that the interstitial defect also exhibits a negative- U behavior. Based on our results, we suggest that the A, B8, and B bands of the local-vibrational-mode absorption spectrum of oxygen in GaAs are due to the off-center OAs defect. Also we show that this spectrum cannot be associated with an interstitial-oxygen configuration as previously proposed.

Using the first-principles pseudopotential approach, we have performed calculations of the total energy, atomic structure, and chemical bonding for the bulk and surfaces of GaAs and cubic GaN to assess the role of the generalized gradient approximation (GGA) for the exchange-correlation energy. Our calculations show that the combined effect of nonlinear core correction and GGA is responsible for significant changes in the structural parameters and electronic band structure of the (001) surface, regardless of the ionicity of the compound studied, but not for bulk or the nonpolar (110) surface.

In this work we have performed an ab initio theoretical study of the In-covered GaAs(001) surface. Our total-energy calculations indicate that for an In ``rich'' environment, with a coverage of 0.75 monolayer, the $\ensuremath{\zeta}(4\ifmmode\times\else\texttimes\fi{}2)$ model represents the energetically most stable structure. The $\ensuremath{\beta}2(4\ifmmode\times\else\texttimes\fi{}2)$ model is dismissed based upon energetic analysis and theoretical scanning tunneling microscopy (STM) images. It is argued that the $\ensuremath{\alpha}$ model is not expected to be stabilized, in contrast to the interpretation of recent STM images.

Using a first-principles pseudopotential method we have studied the adsorption and dissociation of the common n-type dopant molecule ${\mathrm{PH}}_{3}$ on the Si(001) surface. We have found that for low phosphorus coverages $(\frac{1}{4}$ monolayer) phosphine adsorbs molecularly on one side of the Si-Si dimer and, at temperatures around 140 K, fully dissociates into ${\mathrm{PH}}_{2}$ and H, with each component attached to one side of the dimer. For higher phosphorus coverages $(\frac{1}{2}$ monolayer) the interaction between adjacent dimers plays a decisive role in the dissociation process. For both coverages, the surface is characterized by an elongated dimer, symmetric for the dissociated case and asymmetric for the molecular case. The H-P-H angles and H-P bond lengths for the dissociative case are very close to those obtained for the ${\mathrm{PH}}_{3}$ molecule. However, for the molecular case, while the H--P bond length is close to that observed for the ${\mathrm{PH}}_{3}$ molecule, the H-P-H angle is $\ensuremath{\sim}8%$ bigger. Available experimental scanning tunneling microscopy image results are reinterpreted using theoretical images for the model provided in this work. Our dissociative adsorption model is further supported by our calculated vibrational modes, which are in good agreement with available experimental work.

The first application of the recently proposed variational cellular method (VCM) to periodic structures is reported. The method is applied to the determination of the electronic structure of metallic sodium. The results of these calculations, when compared with previous results obtained through other methods, lead to the conclusion that VCM is an accurate and fast method. Empty-lattice energy eigenvalues for the $\ensuremath{\Delta}$ symmetry line in a body-centered-cubic lattice are found to be correct to 0.001 Ry, when spherical harmonic components up to ${l}_{max}=4$ are used in the cellular expansions.

A local approximation to the exchange-correlation effects is used in a detailed calculation of the $s$-core binding-energy splittings for the $3d$ transition elements and rare-earth series. Our approach to the many-body problem is a single-particle model based on an effective wave equation with the mass operator instead of the simpler exchange operator of Hartree-Fock theory. By treating the exchange-correlation effects (mass operator) in a form of an effective local potential in the free-electron gas, the splittings are calculated in the framework of the self-consistent-field method applied to free ions. The results are in good agreement with x-ray photoemission experimental data taken for $3d$ transition and rare-earth ionic compounds and metals.