Periodic ab initio estimates of the dispersive interaction between molecular nitrogen and a monolayer of hexagonal BN

The ab initio determination of the leading long-range term of pairwise additive dispersive interactions, based on the independent analysis of the response properties of the interacting objects, is here considered in the case where these are part of a periodic system. The interaction of a nitrogen molecule with a thin film of hexagonal BN has been chosen as a case study for identifying some of the problems involved, and for proposing techniques for their solution. In order to validate the results so obtained, the interaction energy between N2 and a BN monolayer at different distances has been estimated following a totally different approach, namely by performing post-Hartree–Fock (MP2) supercell calculations using the CRYSTAL+CRYSCOR suite of programs. The results obtained with the two approaches closely agree over a long range, while the limit of validity of the purely dispersive regime can be clearly assessed

An Ab-initio Hartree-fock Study of Electronic and Structural-properties of Mgh2

A periodic ab initio Hartree-Fock method has been used to evaluate a number of electronic and structural properties of MgH2. The calculated quantities are the equilibrium lattice parameters, the cohesive energy, the elastic constants, the energy-band structure, 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 fully ionic character of MgH2 is confirmed by the present study

Comparison between Hartree-Fock and Kohn-Sham electronic and structural properties for hexagonal-close-packed magnesium

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

DL_POLY_2 adaptations for solvation studies

International audienceAdaptations of the mol. dynamics engine DL_POLY\₂ were recently made to facilitate the investigation of questions arising mainly in the study of solvation dynamics. This paper presents the new facilities, with consideration of practical points for their efficient use and illustrative results obtained with them. Supplementary Information provides further details of their implementation to encourage future improvements and extensions. The adaptations of DL_POLY\₂ for solvation studies are: species by species and inter-species breakdown of the total energy for systems contg. several species, such as solns. or models of adsorption, with, as an illustration, diffusion of a particular water mol. in a microporous silica zeolite, silicalite. The new facilities allow correlation of the trajectory of the mol. with its interaction with the host matrix. Solvation-induced spectral shifts, illustrated by the absorption shift and Stokes shift of a cyanoarom. dye in soln. in methanol. We discuss the relative importance of different contributions to the total spectral shift. Free energies by thermodn. integration, illustrated by calcn. of the excess free energy of liq. dimethylsulfoxide. We examine the use of different weighting functions for the mixed Hamiltonian to avoid or to delay divergence in thermodn. integration

Sensitivity of Asphaltene Aggregation toward the Molecular Architecture under Desalting Thermodynamic Conditions

The challenges faced by the oil and shale industries include the comprehension of the physical–chemical behavior of heavy-weight phases. These phases have a high tendency to aggregate, mainly as a result of the presence of asphaltenes, the composition of which has been fairly unclear up to now. The chemical composition of this phase is one of the driving forces behind the physical–chemical behavior in oil, and the structure–property relations of these systems are key in the development of improved refining techniques, including the design of new catalysts. In this paper, the aggregation of asphaltene molecules is studied with regard to molecular architecture and variations in the size of the aromatic core and lateral chain length using classical molecular dynamics simulations. How these characteristics are impacted by the temperature and pressure is also examined. This analysis provides a general overview of the factors that have the strongest impact on the formation and stability of nanoaggregates and clusters of nanoaggregates

Discerning Inter- and Intramolecular Vibrations of Sulfur Polyaromatic Compounds

Thiophenes are an important class of molecules in fields as diverse as petrochemistry, molecular electronics, and optoelectronics. Thiophenic submolecular motifs are thought to play a role in molecular association and nanoaggregation phenomena in both pure materials and natural and synthetic mixtures. Vibrational (infrared and Raman) spectroscopy provides the means to characterize these species. In this work far-infrared photoacoustic and low-frequency Raman spectra of a series of polycyclic aromatic hydrocarbons containing sulfur have been measured and interpreted using DFT calculations based on a perturbational-variational method coupled with potential truncation. The approach and outcomes illustrate how inter- and intramolecular vibrations for thiophenic systems in single and multicomponent mixtures can be discriminated. This work offers the perspective to search the inter- and intramolecular signatures of the main submolecular motifs and heteroelements postulated as being present in the asphaltenes

First principles studies of SnTiO3 perovskite as a potential ferroelectric material

In the context of the search for environment-respectful, lead- and bismuth- free chemical compounds for devices such as actuators, SnTiO3 (ST) is investigated from first principles within DFT. Full geometry optimization provides a stable tetragonal structure relative to cubic one. From the equation of state the equilibrium volume of SnTiO3 is found smaller than ferroelectric PbTiO3 (PT) in agreement with a smaller Sn2+ radius. While ionic displacements exhibit similar trends between ST and PT a larger tetragonality (c/a ratio) for ST results in a larger polarization, PST = 1.1 C.m2. The analysis of the electronic band structure detailing the Sn-O and Ti-O interactions points to a differentiated chemical bonding and a reinforcement of the covalent bonding with respect to Pb homologue.Comment: publishes in Chemical Physics 355 (2009) 43-4