Mauro Causà

Abinitio approach to molecular crystals: A periodic Hartree–Fock study of crystalline urea

The electronic structure of crystalline urea (two molecules, 16 atoms per unit cell) is investigated at an ab initio level with CRYSTAL, a Hartree–Fock linear combination of atomic orbitals (LCAO) program for periodic systems. The influence of the basis set and of the computational parameters which control the treatment of the Coulomb and exchange series and the reciprocal space integration is documented; results include total and interaction energy, Mulliken analysis data and interaction (solid minus molecules) density maps, band structure, and density of states. The crystal field modifies the electronic structure of the isolated molecule, the main effect being an increase in the ionicity of bonds. The interaction energy obtained with a 6‐21** basis set is 28 kcal/mol, (16 kcal/mol after a correction of the basis set superposition error by using the counterpoise method) to be compared with 21±0.5 kcal/mol from experiment. This preliminary application shows that accurate ab initio calculations of hydrogen...

AB initio hartree-fock study of the MgO(001) surface

Abstract The MgO(001) surface has been studied with an ab initio Hartree-Fock crystalline orbital LCAO program. An optimized basis set containing nine atomic orbitals (three s and six p) per atom has been used. The semi-infinite crystal has been simulated by a slab containing three planes (six atoms per cell). In agreement with the most recent LEED experiments, no relaxation is found and the “rumpling” is very small (1% of the nearest neighbour separation in the bulk). The analysis of the electron charge distribution and of the ion multipoles shows that the fully ionic character found for the bulk is maintained at the surface, and that the anion deformation is very small. A surface energy of 1.43 J/m 2 was obtained.

Density functional theory in periodic systems using local Gaussian basis sets

This paper describes a new implementation of density functional theory for periodic systems in a basis of local Gaussian functions, including a thorough discussion of the various algorithms. The results of test calculations using this scheme are reported for some representative crystalline solids, with particular emphasis placed on the influence of the various computational parameters used to control the accuracy of the numerical integration of the exchange-correlation potential.

Ab initio characterization of the (0001) and (101̄0) crystal faces of α-alumina

Abstract Ab initio Hartree-Fock calculations of the (0001) and (1010) crystal faces of α-alumina have been performed using a slab model and taking full account of two-dimensional periodicity. Surface relaxation, surface formation energy, Mulliken populations, charge density maps, and densities of states are obtained and discussed. The validity of the slab model is discussed by analyzing the dependence of results on slab thickness.

Density functional study of oxygen vacancies at the SnO2 surface and subsurface sites

Oxygen vacancies at the SnO2(110) and (101) surface and subsurface sites have been studied in the framework of density functional theory by using both all-electron Gaussian and pseudopotential plane-wave methods. The all-electron calculations have been performed using the B3LYP exchange-correlation functional with accurate estimations of energy gaps and density of states. We show that bulk oxygen vacancies are responsible for the appearance of a fully occupied flat energy level lying at about 1 eV above the top valence band, and an empty level resonant with the conduction band. Surface oxygen vacancies strongly modify the surface band structures with the appearance of intragap states covering most of the forbidden energy window, or only a small part of it, depending on the vacancy depth from the surface. Oxygen vacancies can account for electron affinity variations with respect to the stoichiometric surfaces as well. A significant support to the present results is found by comparing them to the available experimental data.

A theoretical study of stability, electronic, and optical properties of GeC and SnC

We present the results of a first principles study on the ordered Ge 0.50 C 0.50 and Sn 0.50 C 0.50 cubic alloys. A linear combination of atomic orbitals approach in the framework of density functional theory is employed for total energy calculations in the zincblende phase. A fitting of the energy surface to the equation of state yields the lattice constant of 4.61 and 5.17 A and the bulk modulus of 181 and 119 GPa for GeC and SnC, respectively. Analysis of band structure suggests a crossover of the nature of the band gap from indirect to direct in going from SiC to GeC to SnC. Although both alloys predicted to be unstable with respect to their elemental components at zero pressure and temperature, GeC appears to become stable at higher pressure. It appears that both the lattice

A periodic ab initio extended basis set study of α-Al2O3

Corundum, or α-Al2O3, is investigated at the ab initio periodic Hartree-Fock LCAO level using the program CRYSTAL. A split valence 6–21G basis set is adopted, and the influence of d polarization functions is explored. The four geometrical parameters of the unit cell are optimized, the difference with respect to the experimental data always being less than 1%. The electric field gradient tensor at both nuclei (Al and O) is calculated; agreement with the experimental data is quite satisfactory. Band structure, density of states and charge density maps are also given. The resulting electronic structure is highly ionic.

Density functional LCAO calculation of periodic systems. A posteriori correction of the Hartree-Fock energy of covalent and ionic crystals

Abstract Numerous density functional theory formulas proposed in the literature are used to compute the total electronic energy of solids, integrating a posteriori functionals of electronic density, obtained with the periodic linear combination of the atomic orbitals Hartree-Fock method. Formation energies, equilibrium geometries and bulk moduli are calculated for a set of 18 covalent, ionic and semi-ionic crystals. A set of 11 density functional models is used. The calculated observables are compared with experimental data; a statistical analysis is performed to compare the models with greater ease.

Density-Gradient Analysis for Density Functional Theory: Application to Atoms*

We present an analysis of local or semilocal density functionals for the exchange)correlation energy by decomposing them into their gradients r local Seitz s . . . radius , z relative spin polarization , and s reduced density gradient . We explain the numerical method pertaining to this kind of analysis and present results for a few atoms and ions. The atomic shell structure is prominent, and only the ranges 0 - r - 10 and s 0 - s - 3 are important. The low-density and large-gradient domains, where the approximations for the exchange)correlation energy are least trustworthy, have very little weight. Q 1997 John Wiley & Sons, Inc.

A periodic ab initio Hartree-Fock calculation on corundum

Abstract The electronic structure of corundum (α-Al 2 O 3 ) is calculated at the ab initio Hartree-Fock level. Cohesive energy, total and projected densities of states, atomic multipoles, bond populations and electron charge density distribution maps are given. The oxygen-aluminium bond is found to be partially covalent in nature; the atomic charges are −0.73 e and +1.09 e for O and Al respectively.