Potential models for the simulation of methane adsorption on graphene: development and CCSD(T) benchmarks.

Different force fields for the graphene-CH4 system are proposed including pseudo-atom and full atomistic models. Furthermore, different charge schemes are tested to evaluate the electrostatic interaction for the CH4 dimer. The interaction parameters are optimized by fitting to interaction energies at the DFT level, which were themselves benchmarked against CCSD(T) calculations. The potentials obtained with both the pseudo-atom and full atomistic approaches describe accurately enough the average interaction in the methane dimer as well as in the graphene-methane system. Moreover, the atom-atom potentials also correctly provide the energies associated with different orientations of the molecules. In the atomistic models, charge schemes including small charges allow for the adequate representation of the stability sequence of significant conformations of the methane dimer. Additionally, an intermediate charge of -0.63e on the carbon atom in methane leads to bond energies with errors of ca. 0.07 kcal mol-1 with respect to the CCSD(T) values for the methane dimer. For the graphene-methane interaction, the atom-atom potential model predicts an average interaction energy of 2.89 kcal mol-1, comparable to the experimental interaction energy of 3.00 kcal mol-1. Finally, the presented force fields were used to obtain self-diffusion coefficients that were checked against the experimental value found in the literature. The no-charge and Hirshfeld charge atom-atom models perform extremely well in this respect, while the cheapest potential considered, a pseudo-atom model without charges, still performs reasonably well.

Adsorption of Hydrogen Molecules on Carbon Nanotubes Using Quantum Chemistry and Molecular Dynamics.

Physisorption and storage of molecular hydrogen on single-walled carbon nanotube (SWCNT) of various diameters and chiralities are studied by means of classical molecular dynamics (MD) simulations and a force field validated using DFT-D2 and CCSD(T) calculations. A nonrigid carbon nanotube model is implemented with stretching (C-C) and valence angle potentials (C-C-C) formulated as Morse and Harmonic cosine potentials, respectively. Our results evidence that the standard Lennard-Jones potential fails to describe the H2-H2 binding energies. Therefore, our simulations make use of a potential that contains two-body term with parameters obtained from fitting CCSD(T)/CBS binding energies. From our MD simulations, we have analyzed the interaction energies, radial distribution functions, gravimetric densities (% wt), and the distances of the adsorbed H2 layers to the three zigzag type of nanotubes (5,0), (10,0), and (15,0) at 100 and 300 K.

Energy transfer upon collision of selectively excited CO2 molecules: State-to-state cross sections and probabilities for modeling of atmospheres and gaseous flows.

Carbon dioxide molecules can store and release tens of kcal/mol upon collisions, and such an energy transfer strongly influences the energy disposal and the chemical processes in gases under the extreme conditions typical of plasmas and hypersonic flows. Moreover, the energy transfer involving CO2 characterizes the global dynamics of the Earth-atmosphere system and the energy balance of other planetary atmospheres. Contemporary developments in kinetic modeling of gaseous mixtures are connected to progress in the description of the energy transfer, and, in particular, the attempts to include non-equilibrium effects require to consider state-specific energy exchanges. A systematic study of the state-to-state vibrational energy transfer in CO2 + CO2 collisions is the focus of the present work, aided by a theoretical and computational tool based on quasiclassical trajectory simulations and an accurate full-dimension model of the intermolecular interactions. In this model, the accuracy of the description of the intermolecular forces (that determine the probability of energy transfer in molecular collisions) is enhanced by explicit account of the specific effects of the distortion of the CO2 structure due to vibrations. Results show that these effects are important for the energy transfer probabilities. Moreover, the role of rotational and vibrational degrees of freedom is found to be dominant in the energy exchange, while the average contribution of translations, under the temperature and energy conditions considered, is negligible. Remarkable is the fact that the intramolecular energy transfer only involves stretching and bending, unless one of the colliding molecules has an initial symmetric stretching quantum number greater than a threshold value estimated to be equal to 7.

Ion size influence on the Ar solvation shells of M(+)-C6F6 clusters (M = Na, K, Rb, Cs).

The size-specific influence of alkali metal ions in the gradual transition from cluster rearrangement to solvation dynamics is investigated by means of molecular dynamics simulations for alkali metal cation-hexafluorobenzene systems, M(+)-C(6)F(6) (M = Na, K, Rb and Cs), surrounded by Ar atoms. To analyze such transition, different small aggregates of the M(+)-C(6)F(6)-Ar(n) (n = 1, ..., 30) type and M(+)-C(6)F(6) clusters solvated by about 500 Ar atoms are considered. The Ar-C(6)F(6) interaction contribution has been described using two different formalisms, based on the interaction decomposition in atom-bond and in atom-effective atom terms, which have been applied to study the small aggregates and to investigate the Ar solvated M(+)-C(6)F(6) clusters, respectively. The selectivity of the promoted phenomena from the M(+) ion size and their dependence from the number of Ar atoms is characterized.

Ar solvation shells in K(+)-HFBz: from cluster rearrangement to solvation dynamics.

The effect of some leading intermolecular interaction components on specific features of weakly bound clusters involving an aromatic molecule, a closed shell ion, and Ar atoms is analyzed by performing molecular dynamics simulations on potential energy surfaces properly formulated in a consistent way. In particular, our investigation focuses on the three-dimensional Ar distributions around the K(+)-hexafluorobenzene (K(+)-HFBz) dimer, in K(+)-HFBz-Ar(n) aggregates (n ≤ 15), and on the gradual evolution from cluster rearrangement to solvation dynamics when ensembles of 50, 100, 200, and 500 Ar atoms are taken into account. Results indicate that the Ar atoms compete to be placed in such a way to favor an attractive interaction with both K(+) and HFBz, occupying positions above and below the aromatic plane but close to the cation. When these positions are already occupied, the Ar atoms tend to be placed behind the cation, at larger distances from the center of mass of HFBz. Accordingly, three different groups of Ar atoms are observed when increasing n, with two of them surrounding K(+), thus, disrupting the K(+)-HFBz equilibrium geometry and favoring the dissociation of the solvated cation when the temperature increases. The selective role of the leading intermolecular interaction components directly depending on the ion size repulsion is discussed in detail by analyzing similarities and differences on the behavior of the Ar-solvated K(+)-HFBz and Cl(-)-Bz aggregates.

A portable intermolecular potential for molecular dynamics studies of NMA-NMA and NMA-H2O aggregates.

A recently formulated intermolecular potential has been adapted to describe the interaction of the N-methylacetamide (NMA) dimer and of the NMA-H(2)O adduct. The pure electrostatic component of the intermolecular potential functional representation is as usual expressed in terms of a set of punctual charges distributed over the molecular frames, consistently with the permanent molecular dipole values. In contrast, the remainder of the intermolecular potential is expressed in terms of Improved Lennard Jones effective pair potential functions, referred to multiple interaction centers (or sites) placed on the N-methylacetamide molecule and to a single interaction center placed on the water molecule. The characteristic of this pair potential relies on a mix of transferable and non-transferable descriptions of the parameters. The first set of parameters has a structural connotation bearing a site-site interaction nature and exploiting the molecular polarizability decomposability. The second one, depending on the particles clustering and charge distribution and transfer, bears a delocalized and ambient bulk nature. This choice has been tested against ab initio calculations and molecular dynamics simulations. The results show that the model potential is appropriate for describing the energetic of the various stable configurations of NMA-NMA and NMA-H(2)O weakly interacting aggregates, including the formation of hydrogen bonds.