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

On the suitability of the ILJ function to match different formulations of the electrostatic potential for water-water interactions

Energetic and structural properties of liquid water have been calculated using molecular dynamics simulations in order to investigate the effect of different formulations of the van der Waals (vdW) interaction on the behaviour of liquid water. In particular, two model potentials, the SPC/E using a Lennard Jones (LJ) function and the AMPF using an Improved Lennard Jones function (ILJ) have been considered. The flexibility of the ILJ function in the AMPF model has also been analysed, proving that its vdW component can match different parametrizations of the electrostatic component.

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

Silicon-bearing molecules in the shock L1157-B1: first detection of SiS around a Sun-like protostar

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Silicon-bearing molecules in the shock L1157-B1: First detection of SiS around a Sun-like protostar

The shock L1157-B1 driven by the low-mass protostar L1157-mm is a unique environment to investigate the chemical enrichment due to molecules released from dust grains. IRAM-3()m and Plateau de Bure Interferometer observations allow a census of Si-bearing molecules in LI 157-B1. We detect SiO and its isotopologues and. for the first time in a shock. SiS. The strong gradient of the [SiO/SiS| abundance ratio across the shock (from ≥ 180 to ~25) points to a different chemical origin of the two species. SiO peaks where the jet impacts the cavity walls ([SiO/H] ~ 2H)10), indicating that SiO is directly released from grains or rapidly formed from released Si in the strong shock occurring at this location. In contrast, SiS is only detected at the head of the cavity opened by previous ejection events ([SiS/H]~2 x l0). This suggests that SiS is not directly released from the grain cores but instead should be formed through slow gas-phase processes using part of the released silicon. This rinding shows that Si-bearing molecules can be useful to distinguish regions where grains or gas-phase chemistry dominates.This work was supported by the French program ‘Physique et Chimie du Milieu Interstellaire’ (PCMI) funded by CNRS and CNES and by a grant from LabeX Osug@2020 (Investissements d’avenir – ANR10LABX56). This study is based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain)