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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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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%.

Here we report the solid-state experimental and computational analysis of naphtho[2,3-c][1,2,5]thiadiazole (1) and naphtho[2,3- c][1,2,5]selenadiazole (2) showing that the three different crystal phases of polymorphic 1 and the sole crystal phase of nonpolymorphic 2 are characterized by predominance of some intermolecular motifs over the others. In particular, the intermolecular interactions present in the compounds, π···π, C−H···π, C−H···N, S···N, and Se···N, are hierarchically ranked, in such a way that in 1 polymorphism is observed, with the three different phases showing different combinations of the intermolecular interactions, while in 2 only the packing that maximizes the strongest intermolecular interaction by far, i.e., chalcogen bond, is observed. Moreover we also show that different packings produce different responses in the solid-state topochemical reactivity of the crystalline compounds, spanning from nonreactive packings to packings producing butterfly dimers. In order to rationalize the different responses, the relevance of the transverse parallel shift of molecules is suggested, as an important factor, in addition to the known Schmidt’s rules for solid-state topochemical reactions.

The structural, electronic and vibrational properties of two common defects in diamond, CHN and CHB, describing the case in which a carbon C1 atom is substituted by a nitrogen atom, or by a boron atom, breaking a C1–C2 bond, followed by the saturation of the dangling bond of C2 by a hydrogen atom, are investigated at the quantum mechanical level, by using a periodic supercell approach, hybrid DFT functionals and a local Gaussian-type basis set as implemented in the CRYSTAL code. The effect of concentration of the defects has been explored, by considering two supercells containing 64 and 216 atoms (S64 and S216). Formation and hydrogenation energies, geometries, Mulliken charges and the band structure of both defects are reported. The vibrational features of the defects have been investigated, by generating the IR and Raman spectra, and by analyzing graphically and through the isotopic substitution (H → D, 11B → 10B and 14N → 15N) the nature of the most relevant modes related to the defects. The computed C–H stretching mode of CHN, once corrected for anharmonicity (3408 cm $$^{-1}$$ ), falls to wavenumbers very close to the experimental peak observed at 3394 cm $$^{-1}$$ , which can then be reasonably attributed to this specific defect. The present manuscript is included in a special volume in honor and memory of Janos Angyan. Although he did not study in particular the kind of defects discussed in the present manuscript, the many methodological contributions he introduced in computational science have inspired many of the tools we have been using here. One of the present authors, RD, in particular, is grateful to Janos for the illuminating discussions they had in Paris, Nancy and Torino.

cited By 0; International audience; The VN3H defect in diamond (a vacancy surrounded by three nitrogen and one carbon atoms, the latter being saturated by a hydrogen atom) is investigated quantum-mechanically by use of a periodic supercell approach, an all-electron Gaussian-type basis set, "hybrid" functionals of density functional theory, and the Crystal program. Three fully optimized structural models (supercells containing 32, 64, and 128 atoms) are considered to investigate the effect of defect concentration. The electronic configuration of the defect is reported along with a description of its structural features. In particular, the influence of the lone-pair electrons of the three nitrogen atoms on the C-H bond is discussed. A thorough characterization of the vibrational spectroscopic features of the VN3H defect is also presented, where the anharmonicity of the most relevant normal modes is discussed. The infrared and Raman spectra show specific peaks, which allow for the identification of this particular defect among the many defects that are commonly present in both natural and irradiation-damaged diamonds. In particular, the main feature of the spectral fingerprint of the defect (i.e. the C-H stretching mode), experimentally observed at 3107 cm-1, is here computed at 3094 cm-1 with the B3LYP "hybrid" functional (with an anharmonic redshift of 157 cm-1 with respect to its harmonic value). The role played by the three nitrogen atoms on the spectral features of the defect is clearly identified through the redshift due to the 14N → 15N isotopic substitution. © 2017 the Owner Societies.

The adsorption of TiCl4 on the surfaces of MgCl2 crystals has been investigated by means of state-of-the-art periodic hybrid DFT methods, as the first step of a comprehensive study aiming to elucidate the structure of the active species in industrial MgCl2-supported Ziegler-Natta catalysts for ethene and propene polymerization. A first distinctive feature of the approach was the thorough evaluation of dispersion forces, crucial because the binding of TiCl4 on MgCl2 surfaces turned out to be essentially dispersion-driven. Also important was a careful investigation of the effects of different choices on basis set and density functional (DF) on the quantitative aspects of the results; this allowed us to trace the unusually large disagreement in the previous literature and identify unambiguous trends. In particular, three full sets of calculations were run adopting the B3LYP(-D), PBE0(-D) and M06 approximations: to the best of our knowledge, the last represents the first case of M06 functional implementation in a periodic code (CRYSTAL) of widespread use. The results consistently indicated that the adsorption of TiCl4 on well-formed MgCl2 crystals under conditions relevant for catalysis can only occur on MgCl2(1 1 0) or equivalent lateral faces, whereas the interaction with MgCl2(1 0 4) - for decades claimed as the most important surface in stereoselective catalysts - is too weak for the formation of stable adducts. The implications of these findings for catalysis are discussed. (C) 2011 Elsevier Inc. All rights reserved.

Activated MgCl2 nanocrystals were prepared by controlled dealcoholation of the MgCl2 center dot 6CH(3)OH adduct, mimicking the routinely adopted methods to synthesize industrial heterogeneous Ziegler Natta catalysts. The effect of the alcohol in driving the morphology of MgCl2 crystals, i.e., the type and extension of the exposed surfaces, was investigated by integrating a detailed structural, morphological, and surface characterization with a state-of-the-art computational modeling. FT-IR spectroscopy of CO adsorbed at 100 K emerged as a feasible, simple, and powerful method to characterize the surface of structurally disordered MgCl2 and MgCl2-based Ziegler Natta catalysts. Our computational morphological analysis revealed that the (012), (015), and (110) surfaces are highly stabilized donor, especially at the temperature typically adopted in the preparation of the precatalysts. FT-IR spectroscopy of adsorbed CO allows distinguishing these surfaces from the other penta-coordinated ones and provides a clear experimental evidence that TiCl4 binds to the (110) and (015) surfaces. The (015) surface was never considered in the past and is characterized by an unusual flexibility in the presence of adsorbates, which detach the Mg cations from the Cl underneath, leaving a coordination vacancy available for the binding of asymmetric titanium sites. Since the recent literature identified a tetra-coordinated Mg as a site of election for the deposition of the Ti species relevant in olefin polymerization, the presence of two eligible Mg sites for Ziegler Natta catalysis is highly interesting.

Maximum Probability Domain (MPD) analysis has been recently applied to pure covalent and ionic crystals. The present study is devoted to a first MPD analysis of semi ionic crystals, Silicon Oxide, Aluminum Oxide and Titanium Oxide. These crystals are involved in important catalytic and photo-catalytic processes occurring on their surfaces. For this reason the study has been performed on bulk crystal and on surface slab models. Also surface neutral oxygen vacancy, the F-0 surface defect, has been considered. The Electron Localization Function (ELF) analysis has also been performed, due to its holistic approach to electronic structures. (C) 2015 Published by Elsevier BM.

5 pags.; 2 figs.; 2 tabs., In this work we propose a general strategy to calculate accurate He-surface interaction potentials. It extends the dispersionless density functional (dlDF) approach recently developed by Pernal et al. [Phys. Rev. Lett. 109, 263201 (2009)] to adsorbate-surface interactions by including periodic boundary conditions. We also introduce a scheme to parametrize the dispersion interaction by calculating two- and three-body dispersion terms at CCSD(T) level via the method of increments [Stoll, J. Chem. Phys. 97, 8449 (1992)]. The performance of the composite approach is tested on 4He/graphene by determining the energies of the low-lying selective adsorption states, finding an excellent agreement with the best available theoretical data. Second, the capability of the approach to describe dispersionless correlation effects realistically is used to extract dispersion effects in time-dependent density functional simulations on the collision of 4He droplets with a single graphene sheet. It is found that dispersion effects play a key role in the fast spreading of the 4He nanodroplet, the evaporation-like process of helium atoms, and the formation of solid-like helium structures. These characteristics are expected to be quite general and highly relevant to explain experimental measurements with the newly developed helium droplet mediated deposition technique. © 2014 AIP Publishing LLC, This work has been supported by Grants Nos. CCG08-CSIC/ESP-3680 from CSIC-CM, FIS2011-29596-C02-01 and FIS2011-28617- C02-01 from DGI, Spain (FEDER), and 2009SGR1289 from Generatitat de Catalunya.