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Electron Localization Function (ELF) and Maximum Probability Domain (MPD) analyses have been applied to model metal–porphyrins and show compatible and complementary results. ELF basins are quite different from MPDs, but are a necessary starting point for optimizing them. The analyses of the bond between the metal and porphyrin do not show significant differences between nontransition and transition metals. In all the cases considered, we find signatures characteristic of essentially ionic bonds.

We have performed Time Dependent Density Functional Theory (TDDFT) calculations employing a cluster model of the core excitation spectra of vanadium pentoxide, V2O5. The excitation energies and dipole transition moments are determined for all the core edges, vanadium and oxygen K- and vanadium L-edges, treating them at the same level of accuracy. The agreement between the TDDFT theoretical spectra and the experimental data is rather good, particularly at the V and O K-edges. A quantitative reproduction of the fine pre-edge structures appears more difficult for the V L-edge. The comparison between the TDDFT results and the results obtained at the simpler one electron Kohn-Sham (KS) level indicates that the V and O K edges can be correctly described within a single particle approximation (KS), while the strong modi. cation of the V L-edge structures from the KS to the TDDFT description emphasizes the importance of configuration mixing to treat the metal 2p excitations. The origin of the calculated pre-edge features is analyzed in detail with the help of the atom-projected density-of-states of the unoccupied levels. This analysis emphasizes the V 3d dominant character of the final states in the conduction band, probed by the V L-absorption. The strong octahedral distortion of the V2O5 structure allows the mixing of the 3d state with the V 4p components, which are mapped by the oscillator strength in the V K- edge spectrum. The high intensity of the O 1s transitions reflects the presence of a significant O 2p component in the conduction band.

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All-electron density-functional calculations are performed to study atomic structure and electronic properties of the nonpolar surfaces, namely zinc blende (110) and wurtzite (10 (1) over bar 0) of AlN. Both surfaces are modeled using a two-dimensional periodic slab allowing the relaxation of the first two surface layers in the calculations. The results predict a small layer rotation angle accompanied by a contraction of Al-N bond length for both surfaces. These results do not follow the well-accepted rotation-relaxation model that predicts large layer rotation angles (similar to 28 degrees) with no change in the bond length for most of the III-V semiconductor surfaces. Analysis of the relaxed configurations of the AlN surfaces in terms of atomic geometry, density of states, and charge density plots shows a presence of partial double-bond character in the surface Al-N bond. A similarity of these results with an earlier study on GaN nonpolar surfaces [J. E. Jaffe, R. Pandey, and P. Zapol, Phys. Rev. B 53, R4209 (1996)] led us to suggest the contraction-relaxation model where the relaxation proceeds via strengthening of the surface bond. The primary driving force of such a type of relaxation appears to be the ability of nitrogen to form a double bond that facilitates redistribution of the charge density associated with anion dangling bond to the surface bond.

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In this article, the results of a recently implemented DFT a posteriori and Kohn-Sham (Ks) linear combination of atomic orbital computational scheme for solids are presented. The equilibrium lattice parameters, bulk moduli, and lattice energies are calculated for eight crystallized systems. Local density approximation (LDA) and generalized gradient approximation (GGA) functionals and potentials are used. The maps of the Hartree-Fock (HF) and Ks electronic densities and band structures are depicted. The Ks results confirm the trend of the a posteriori scheme. Very good agreement between calculated and experimental lattice energies has been found for GGA potentials. (C) 1995 John Wiley & Sons, Inc.

The Density-Functional method, with Linear Combination of Atomic Orbitals, has been applied to eight crystals: the lattice equilibrium parameters, and the lattice formation energies have been calculated at the Hartree-Fock level (HF), at the hybrid Hartree-Fock Density-Functional level (DFT/HF), and at the Kohn-Sham Density-Functional level (DFT). The band structures and the electronic charge distributions calculated at the DFT and HF levels are compared. (C) 1994 John Wiley and Sons, Inc.

The ground-state crystal energies of cubic MgAl2O4 (spinel) and MgO (periclase) and of rhombohedral alpha-Al2O3 (corundum) have been calculated at different volumes, relaxing the corresponding structures, by all-electron periodic Hartree-Fock methods (CRYSTAL program). Basis sets of contracted Gaussian-type functions are employed for the 18 atomic (including d) orbitals representing each of the Mg, Al, and O atoms. Mulliken net atomic charges z(Mg)=1.86\e\ (MgO), z(Al)=2.30\e\ (alpha-Al2O3), z(Mg)=1.74\e\, and z(Al) = 2.24\e\ (spinel) are obtained. The elastic bulk modulus, the Murnaghan equation of state p(V) at the athermal limit, the Mg-O and Al-O bond compressibilities, and the binding energy have been derived for each phase (and the elastic constants C-11 and C-12 for spinel only). Comparison with existing experimental data is discussed. The enthalpy change for spinel decomposition into the simple oxides has been computed as a function of pressure, including a correction for the electron correlation energy based on local-density-functional theory. A decomposition pressure of 11 GPa at T=0 K is predicted, against values of 8 and 13 GPa derived from experimental thermodynamic data and from direct compression experiments, respectively.

International audience; A periodic ab initio Hartme-Fock method has been used to evaluate a number of electronic and structural properties of MgHs. The calculated quantities are the equilibrium lattice parameters, the cohesive energy, the elastic constants, the energy-band struo ture, the density of' states, the electronic charge distribution and the Compton profiles. For the two former properties correlation effects are taken into account and improve the agreement with the experimental data.The strongly but not fitlly ionic character of MgHs is confirmed by the present study.

A periodic ab initio Hartree-Fock method (the program CRYSTAL) has been used to evaluate the total-electron-energy surface of MgF2 (rutile-type tetragonal structure) as a function of crystal strain. Mg and F atoms are represented by 13 atomic orbitals in the form of contracted Gaussian-type functions. The equilibrium unit-cell edges and fluorine coordinates, the binding energy, and the six elastic constants C-11, C-12, C-13, C33, C44, and C66 have been calculated. Inner strain was accounted for by relaxing the F-atom position for each lattice deformation applied, and contributed significantly to the C44, C66, and C33 components. An average deviation of 8.0% is observed with respect to experimental elastic data Classical two-body empirical calculations have been performed for the purpose of comparison. Energy bands, Mulliken electron populations, and charge-density maps are analyzed, and the chemical bonding is discussed, showing significant deviations from ionicity (z(Mg) = 1.80(e)).

An array of point multipoles is used to mimic the long-range Coulomb interactions in a TiO56- cluster model designed to describe the adsorption of CO on a rutile surface. The multipoles are derived from the electron density and the concomitant electrostatic potential of a slab model of the (110) surface of rutile as calculated in a periodic Hartree-Fock approach. The motivation for the use of a cluster model is the possibility of inclusion of electron correlation by means of quantum-chemical methods, which is to date not easily possible in periodic Hartree-Fock calculations. In contrast to an array of point charges based on a Mulliken population analysis of the periodic charge distribution, the distributed multipoles show good agreement of adsorption properties as compared to a slab calculation. Surface relaxation has a significant influence on the binding of the adsorbate.

The binding energy (BE), the equilibrium lattice parameter (alpha-0), the bulk modulus (B), and the central-zone transverse-optical phonon frequency (v) of seventeen diamond- and zinc-blende-type-structure semiconductors involving atoms belonging to the second to fifth periods have been evaluated. The periodic ab initio Hartree-Fock linear-combination-of-atomic-orbitals program CRYSTAL has been used. Core pseudopotentials have been adopted in order to limit the calculation to valence electrons. Thirteen atomic orbitals (2s, 6p, 5d) per atom have been used. The quality of the pseudo-potential results is checked by comparison with all-electron calculations performed on six light-atom systems (diamond, silicon, BN, BP, AlP, SiC). The mean errors of the pseudopotential calculations with respect to experimental data are -38%, +1.2%, +6.5%, and +7.8% for BE, alpha-0, B, and v, respectively. A correlation-only density-functional alpha-posteriori correction to the Hartree-Fock total energy is performed, which reduces the absolute BE mean error to 2.6%.