A simple model is presented in which the instantaneous dipole moment of the exchange hole is used to generate a dispersion interaction between nonoverlapping systems. The model is easy to implement, requiring no electron correlation (in the usual sense) or time dependence, and has been tested on various atomic and molecular pairs. The resulting C6 dispersion coefficients are remarkably accurate.
"They all have in common that in principle they can be applied to any density functional. One subclass of these approaches are density-dependent corrections , such as the exchange-hole dipole moment (XDM) model by Becke and Johnson      , the DFT+vdW/ vdW-TS method by Tkatchenko and Scheffler  and the density-dependent dispersion correction (dDsC) correction by Steinmann and Corminboeuf . Applications of these methods are numerous and they all showed that these models can provide results with very good accuracy.[33–47] "
[Show abstract][Hide abstract] ABSTRACT: The recent advent of dispersion-corrected density-functional theory (DFT) methods allows for quantitative modelling of molecular self-assembly processes, and we consider what is required to develop applications to the formation of large self-assembled monolayers (SAMs) on hydrophobic surfaces from organic solution. Focus is on application of the D3 dispersion correction of Grimme combined with the solvent dispersion model of Floris, Tomasi and Pascual–Ahuir to simulate observed scanning-tunnelling microscopy (STM) images of various polymorphs of tetraalkylporphyrin SAMs on highly oriented pyrolytic graphite surfaces. The most significant problem is identified as the need to treat SAM structures that are incommensurate with those of the substrate, providing a challenge to the use of traditional periodic-imaging boundary techniques. Using nearby commensurate lattices introduces non-systematic errors into calculated lattice constants and free energies of SAM formation that are larger than experimental uncertainties and polymorph differences. Developing non-periodic methods for polymorph interface simulation also remains a challenge. Despite these problems, existing methods can be used to interpret STM images and SAM atomic structures, distinguishing between multiple feasible polymorph types. They also provide critical insight into the factors controlling polymorphism. All this stems from a delicate balance that the intermolecular D3 and solvent Floris, Tomasi and Pascual–Ahuir corrections provide. Combined optimised treatments should yield fully quantitative approaches in the future.
"In a recent study, it has shown that this election works quiet well for non-equilibrium geometries. The XDM parameters were obtained from  : a1 = 0.136 and a2 = 3.178 Å. The plane wave energy cutoff was selected to 60 Ry (checking that the total energy is converged at that value) and the reciprocal space was divided into a Monkhorst-Pack  grid of 2 × 2 × 2 points. "
[Show abstract][Hide abstract] ABSTRACT: First-principles calculations were performed to determine equilibrium geometries, static equation of state parameters, the energetics and orientation of the guest molecule inside the 5 and 56 cages, and vibrational frequencies of methane clathrate hydrate. According to our results, the progressive inclusion of one molecule in each clathrate cavity is always a stabilizing process up to saturation. The released energy is very similar for both types of cages. In agreement with the experimental observation of roto-vibrational spectra in this hydrate, we calculate an energy barrier of less than 0.5 kcal/mol, indicating free rotation of methane inside the cages. The stabilizing effect of applied pressure leads to a red shift of the O–H stretching frequencies of the water molecules of around 80 cm in average at 1 GPa.
High Pressure Research 01/2015; 35(1). DOI:10.1080/08957959.2014.996560 · 0.95 Impact Factor
"However, since the dispersion forces are always attractive they enhance the cohesive energy of the system (liquid or solid), a densification effect which could remedy to some of the inaccuracies encountered here with the use of GGA BLYP. Several routes (empirical or semi empirical) are proposed in the literature to estimate the dispersion contribution to the interaction energy (Becke and Johnson, 2005, 2007; Grimme, 2006; Civalleri et al., 2008): they are currently under investigation. "
[Show abstract][Hide abstract] ABSTRACT: The structural and dynamical properties of four silicate liquids (silica, rhyolite, a model basalt and enstatite) are evaluated by ab initio molecular dynamics simulation using the density functional theory and are compared with classical simulations using a simple empirical force field. For a given composition, the structural parameters of the simulated melt vary little between the two calculations (ab initio versus empirical) and are in satisfactory agreement with structure data available in the literature. In contrast, ionic diffusivities and atomic vibration motions are found to be more sensitive to the details of the interactions. Furthermore, it is pointed out that the electronic polarization, as evaluated by the ab initio calculation, contributes significantly to the intensity of the infrared absorption spectra of molten silicates, a spectral feature which cannot be reproduced using nonpolarizable force field. However the vibration modes of TO4 species and some structural details are not accurately reproduced by our ab initio calculation, shortcomings which need to be improved in the future.
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