Search

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.

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.

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.

The concepts of atoms and bonds in molecules which appeared in chemistry during the nineteenth century are unavoidable to explain the structure and the reactivity of the matter at a chemical level of understanding. Although they can be criticized from a strict reductionist point of view, because neither atoms nor bonds are observable in the sense of quantum mechanics, the topological and statistical interpretative approaches of quantum chemistry (quantum theory of atoms in molecules, electron localization function and maximum probability domain) provide consistent definitions which accommodate chemistry and quantum mechanics.

The capabilities of the CRYSTAL14 program are presented, and the improvements made with respect to the previous CRYSTAL09 version discussed. CRYSTAL14 is an ab initio code that uses a Gaussian-type basis set: both pseudopotential and all-electron strategies are permitted; the latter is not much more expensive than the former up to the first-second transition metal rows of the periodic table. A variety of density functionals is available, including as an extreme case Hartree-Fock; hybrids of various nature (global, range-separated, double) can be used. In particular, a very efficient implementation of global hybrids, such as popular B3LYP and PBE0 prescriptions, allows for such calculations to be performed at relatively low computational cost. The program can treat on the same grounds zero-dimensional (molecules), one-dimensional (polymers), two-dimensional (slabs), as well as three-dimensional (3D; crystals) systems. No spurious 3D periodicity is required for low-dimensional systems as happens when plane-waves are used as a basis set. Symmetry is fully exploited at all steps of the calculation; this permits, for example, to investigate nanotubes of increasing radius at a nearly constant cost (better than linear scaling!) or to perform self-consistent-field (SCF) calculations on fullerenes as large as (10,10), with 6000 atoms, 84,000 atomic orbitals, and 20 SCF cycles, on a single core in one day. Three versions of the code exist, serial, parallel, and massive-parallel. In the second one, the most relevant matrices are duplicated, whereas in the third one the matrices in reciprocal space are distributed for diagonalization. All the relevant vectors are now dynamically allocated and deallocated after use, making CRYSTAL14 much more agile than the previous version, in which they were statically allocated. The program now fits more easily in low-memory machines (as many supercomputers nowadays are). CRYSTAL14 can be used on parallel machines up to a high number of cores (benchmarks up to 10,240 cores are documented) with good scalability, the main limitation remaining the diagonalization step. Many tensorial properties can be evaluated in a fully automated way by using a single input keyword: elastic, piezoelectric, photoelastic, dielectric, as well as first and second hyperpolarizabilies, electric field gradients, Born tensors and so forth. Many tools permit a complete analysis of the vibrational properties of crystalline compounds. The infrared and Raman intensities are now computed analytically and related spectra can be generated. Isotopic shifts are easily evaluated, frequencies of only a fragment of a large system computed and nuclear contribution to the dielectric tensor determined. New algorithms have been devised for the investigation of solid solutions and disordered systems. The topological analysis of the electron charge density, according to the Quantum Theory of Atoms in Molecules, is now incorporated in the code via the integrated merge of the TOPOND package. Electron correlation can be evaluated at the Moller-Plesset second-order level (namely MP2) and a set of double-hybrids are presently available via the integrated merge with the CRYSCOR program. (C) 2014 Wiley Periodicals, Inc.

International audience; The paper presents maximum probability domains (MPDs). These are regions of the three dimensional space for which the probability to find a given number of electrons is maximal. In order to clarify issues hidden by numerical uncertainties, some simple models are used. They show that MPDs reproduce features which one would expect using chemical intuition. For a given number of electrons, there can be several solutions, corresponding to different chemical situations (e.g. different bonds). Some of them can be equivalent, by symmetry. Symmetry can produce, however, alternative solutions. The models show that MPDs do not exactly partition space, and they can also be formed by disjoint subdomains. Finally, an example shows that a partition of space, as provided by loge theory, can lead to situations difficult to deal with, not present for MPDs.

cited By 48; International audience; We present the results of a first principles study on the ordered Ge 0. 50C 0.50 and Sn 0.50C 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 Å 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 constant and bulk modulus of the ordered alloys do not follow Végard's linear rule, though the calculated dielectric constant of the cubic alloys is approximately the average of the dielectric constant of their elemental components.

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.