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In this work, the GLOB model, an effective and reliable computational approach well suited for ab initio and QM/MM molecular dynamics simulations of complex molecular systems in solution, has been applied to study two representative open-shell systems, the cobalt(II) ion and the glycine radical in aqueous solution, with special reference to their structural and magnetic properties. The main structural features of the solvent cage around the cobalt ion and the hydrogen bonding patterns around the neutral and zwitterionic forms of the glycine radical have been investigated in some detail. The general good agreement with experiments supports the use of the present model to investigate more challenging and biological/technological relevant open-shell systems.

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.

By using the CRYSTAL95 program, ab initio periodic Hartree-Fock (PHF) calculations with the full potential and 6-21G* basis set are applied to interpretation of scanning tunneling microscope (STM) images on hexagonal graphite. Our results show asymmetry similar to the experimental and previous pseudopotential calculations. The dominant feature is the deep hollow in the middle of the carbon hexagon. The three carbon atoms that do not have a neighbor in the second layer appear as small hills, the other three with neighbors in the second layer simply appear as saddle points. Our calculation has been successfully used to reproduce experimental features such as the effect of increasing the magnitude of the bias voltage and the effect of increasing the tip-to-surface separation.

Vibrational spectroscopy and Density Functional Theory (DFT) quantum chemical calculations, have been used to investigate structure and chemical properties of some fluorinated ethers. A good agreement between experimental and theoretical results has been obtained. Interesting conformational behaviour has been observed and interpreted for C-H bonds in difluoromethyl end groups. These effects help in understanding the chemical behaviour of these compounds, since C-H equilibrium bond length and bond dissociation energy (BDE) can be correlated with vibrational properties. (C) 1998 Elsevier Science S.A. All rights reserved.

The wave functions of boron, aluminium and gallium phosphides have been calculated at both the Hartree - Fock and local density approximation levels using the ab initio linear combination of atomic orbitals method implemented in the program CRYSTAL. The Mulliken populations, bandstructure, electron charge densities, and structure factors are evaluated and compared. The chemical bonds are mainly covalent, as revealed by the difference electron charge-density maps obtained by subtracting from the total densities the superposition of the spherical atomic densities. A charge-density accumulation between the nearest neighbours slightly shifted towards the phosphorus appears in the AlP and GaP compounds, whereas an opposite charge transfer from phosphorus to boron is observed in boron phosphide from the Mulliken population analysis.

In carbon materials the mobile π electrons are situated in topologically different circumstances at edge sites, and their π electronic states, essentially controlled by the network structure of sp 2 carbon, may be significantly affected. In this work, we derived topological indications about the reactivity of carbon nanotubes and fullerenes with the hydroxyl radical (OH•), the most important oxidizing species in the troposphere. For each molecular structure, we computed the local softness, the Mulliken charges of the reacting carbons of (n,n) and (n,0) clusters, and their Huckel-type aromaticity rules, as an index to determine topologically independent sites and predicting a certain grade of reactivity of the nanotube and fullerenic carbon atoms. Using local softness, closely related to the energy gap, it was possible to separate the periodical nanotubes in three families according to their reactivity. A connection between the reactivity index ΔE and the topology was established by means of the Fukui integrated function. It resulted that for (n,0) clusters, odd n implies aromaticity, whereas even n, non-aromaticity; (n,n) clusters are in any case non-aromatic. For a better understanding of some experimental results, we also discussed how edge effects can influence topological reactivity due to the increment of the number of benzene rings in some cluster arrangements.

The hydrate crystal lithium hydroxide monohydrate LiOH.H2O has been studied by ab initio periodic Hartree-Fock calculations. The influence of the crystalline environment on the local molecular properties (molecular geometry, atomic charges, electron density, molecular vibrations and deuterium quadrupole coupling constants) of the water molecule, the lithium and hydroxide ions has been calculated. A number of crystalline bulk properties are also presented, optimized crystalline structure, lattice energy and electronic band structure. The optimized cell parameters from calculations with a large basis set of triple-zeta quality differ by only 1-3% from the experimental neutron-determined cell, whereas the STO-3g basis set performs poorly (differences of 5-10%). With the triple-zeta basis also the atomic positions and intermolecular distances agree very well with the experiment. The lattice energy differs by approximately 8% from the experimental value, and by at most 3% when a density-functional electron correlation correction is applied. Large electron-density rearrangements occur in the water molecule and in the hydrogen bond and are in qualitative and quantitative agreement with experimental X-ray diffraction results. The quadrupole-coupling constants of the water and hydroxide deuterium atoms are found to be very sensitive to the O-H bond length and are in good agreement with experimental values when the calculation is based on the experimental structure. The anharmonic O-H stretching vibrations in the crystal are presented and found to be very close to results from calculations on molecular clusters. The electronic band and density-of-states spectra are discussed. Model calculations on a hydrogen fluoride chain were used to rationalize the results.

Calculations are reported of the enthalpies of mixing of MnO/MgO and NiO/MgO based on temperature-dependent lattice statics and dynamics and ab initio periodic Hartree-Fock methodology with a posteriori corrections for electron correlation derived from density-functional theory. For MnO/MgO both methodologies predict enthalpies of mixing that are close to those reported by Hahn and Muan (J. Phys. Chem. Solids, 1961, 19, 338). For NiO/MgO, on the other hand, classical simulations predict positive deviations from ideality in contrast to Hartree-Fock calculations which predict small negative deviations in agreement with the experimental data of Davies and Navrotsky (J. Solid State Chem., 1981, 38, 264).

The effect of the Madelung field corresponding to formal point charges ± e arranged according to the fcc lattice of LiF upon CO, N2 and NO molecules is studied in order to isolate the purely electrostatic interaction from the other forces acting in the related physisorption processes. The polarization which is produced in those molecules by the Madelung field is analyzed in terms of dipole and quadrupole moments, as well as in terms of electron charge distributions. The influence of the electron correlation is taken into account by using the QCISD technique, which requires the semi-infinite lattice being simulated with a suitable cluster of point charges.

Extensive DFT calculations are reported for the carbon centered glycine radical embedded in a-glycine crystal. Combined use of periodic and embedded cluster models leads to accurate structural and magnetic properties. Environmental effects are not negligible for all properties, whereas vibrational averaging effects connected to inversion at the radical center are significant only for C-alpha and H-alpha atoms. The good agreement between computed and experimental ESR parameters confirms the nature of one of the radicals obtained upon irradiation of a-glycine crystals and the reliability of DFT computations properly including both vibrational averaging and environmental effects. (c) 2007 Elsevier B.V. All rights reserved.

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., 9 pages, 10 figures; corrected typos

The adsorption of TiCl 4 on the surfaces of MgCl 2 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 MgCl 2-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 TiCl 4 on MgCl 2 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 TiCl 4 on well-formed MgCl 2 crystals under conditions relevant for catalysis can only occur on MgCl 2(1 1 0) or equivalent lateral faces, whereas the interaction with MgCl 2(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

The potentiality of the time dependent density functional theory (TDDFT) for the description of core excitation spectra (XAS) in transition metal oxides is analyzed, considering the rutile form of TiO(2) as a test case. Cluster models are adopted to mimic the bulk, embedded within an array of point charges to simulate the Madelung potential. All of the edges, titanium and oxygen K and titanium L edges, are considered, and the TDDFT results are compared with the experimental data in order to assess the performance of the theoretical approach in dealing with this complex class of compounds. Satisfactory results have been obtained for the Ti and O K edges, while in the case of the Ti L edge some discrepancies with the experiment are still present. The configuration mixing explicitly included in the TDDFT model strongly influences the distribution of the 2p metal oscillator strength. The origin of the spectral features is investigated with the help of the partial density of the virtual states (PDOS) calculated for each core hole considered, which can be qualitatively compared with the theoretical spectra calculated in the Kohn-Sham one-electron approach.

Bulk platinum and platinum slabs one to four (111) layers thick have been studied using the HF and DFT CO-LCAO (Hartree-Fock and density functional theory crystalline-orbital and linear-combination-of-atomic-orbitals) CRYSTAL program. The objective of this paper is to test the suitability of slabs of different thicknesses for modelling the (111) surface of platinum. The cohesive properties of the bulk (lattice constant, bulk modulus, cohesive energy) are reported and the electronic structure of the platinum bulk is discussed. The surface and relaxation energies as well as the relaxation distances of the slabs were evaluated. The electronic structure of the slabs is discussed in terms of Mulliken population analysis, the density of states, the band structure and electron-density difference maps. `Initial-state' surface-atom core-level shifts are reported, as well as atomic dipole and quadrupole moments perpendicular to the surface.

The atomic and electronic structure of the Ag/MgO(100) interface are calculated by means of the ab initio Hartree-Fock approach combined with a supercell model. The electronic density distribution and the interface binding energy/distance are analyzed for different Ag adsorption positions, slabs of different thicknesses and varying Ag surface coverage. It is demonstrated that the adhesion energy arises mainly due to the electrostatic interaction of substrate atoms with a complicated charge redistribution in the metal layer(s), characterized by large quadrupole moments as well as electron density redistribution towards bridge and hollow positions between the nearest and next-nearest Ag atoms. (C) 1998 Elsevier Science B.V.

International audience; The properties of hcp magnesium are investigated using the density functional method with the linear combination of atomic Orbitals as implemented in the CRYSTAL95 code. The lattice equilibrium parameters and the binding energy have been calculated at the Hartree-Fock level, at the hybrid Hartree-Fock density functional level, and at the Kohn-Sham density functional level using local and non-local exchange and correlation potentials. The electronic properties (band structures, topologies of the Fermi surface, and densities of states) and the elastic constants are computed for each type of functional, and compared to experimental data.

The generalization of Becke’s numerical integration scheme to periodic functions is presented, which allows the LCAO-Kohn-Sham equations for crystals to be solved efficiently. The computational implementation of the scheme and its calibration are briefly discussed.

This paper presents results of pseudopotential Hartree-Fock calculations on the titanium dioxide crystals. The optimized structures are in good agreement with experimental data. Both anatase and rutile structures have a large ionic character. The distortions from the undistorted crystals made of regular octahedra are discussed and qualitative trends for the distortions are justified by semiempirical calculations (extended H\"uckel theory). When correlation is taken into account, a large part of the binding energy is recovered and the rutile structure is found to be more stable than the anatase one.

Corundum, or alpha-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 stages and charge density maps are also given. The resulting electronic structure is highly ionic.

The regular adsorption of CO at various concentrations on an infinite three-layer slab of LiF has been studied by using the CRYSTAL ab initio program. The CO molecules are adsorbed vertically above the surface cations by purely electrostatic attraction counterbalanced by the Pauli repulsion. The induced polarization has been analysed through the changes occurring in the molecular orbitals of the adsorbed species. The adsorption via the oxygen atom appears to be preferred, which contradicts IR experimental data. Such contradiction arises from the correlation error in the Hartree-Fock SCF method which is applied in the CRYSTAL program. Consideration of simple correlated clusters enables a semi-quantitative evaluation which additively corrects the above contradictory results.

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.

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.

The core pseudopotentials of Durand and Barthelat have been introduced in the periodic Hartree-Fock program CRYSTAL. This new option allows the treatment of crystals involving heavy atoms and therefore extends the applicability of the method. A comparison between all-electron and pseudopotential results is presented. Binding energies, optimized cell parameters, bulk moduli, band structures and electron density curves calculated with the two methods are presented for covalent (diamond and silicon), metallic (lithium) and ionic (MgO) systems. As expected the pseudopotential results for covalent compounds are in good agreement with those obtained from all-electron calculations; for MgO larger discrepancies are observed, possibly due to core polarization effects.