An intermolecular pair potential energy function for methane

An automated calculation pipeline for differential pair interaction energies with molecular force fields using the Tinker Molecular Modeling Package

An automated pipeline for comprehensive calculation of intermolecular interaction energies based on molecular force-fields using the Tinker molecular modelling package is presented. Starting with non-optimized chemically intuitive monomer structures, the pipeline allows the approximation of global minimum energy monomers and dimers, configuration sampling for various monomer-monomer distances, estimation of coordination numbers by molecular dynamics simulations, and the evaluation of differential pair interaction energies. The latter are used to derive Flory-Huggins parameters and isotropic particle-particle repulsions for Dissipative Particle Dynamics (DPD). The computational results for force fields MM3, MMFF94, OPLS-AA and AMOEBA09 are analyzed with Density Functional Theory (DFT) calculations and DPD simulations for a mixture of the non-ionic polyoxyethylene alkyl ether surfactant C10E4 with water to demonstrate the usefulness of the approach.Scientific ContributionTo our knowledge, there is currently no open computational pipeline for differential pair interaction energies at all. This work aims to contribute an (at least academically available, open) approach based on molecular force fields that provides a robust and efficient computational scheme for their automated calculation for small to medium-sized (organic) molecular dimers. The usefulness of the proposed new calculation scheme is demonstrated for the generation of mesoscopic particles with their mutual repulsive interactions.

Investigations on the Structure of Fluid Methane by Neutron Diffraction Experiments

High pressure neutron diffraction experiments on a mixture of CH4 and CD4 as well as on pure deuterated methane CD4 were performed at three supercritical states. The structure factor was determined at three different densities at a temperature of T = 370K. The intra- and intermolecular structure was determined from the structure factor. The results from pure CD4 are in very good agreement with former neutron diffraction experiments. The experiments on the isotopic mixture allowed the direct determination of the pair correlation function of the molecular centres. The results of the diffraction experiments are presented in this paper.

Infrared Spectra and Ab Initio Calculations for the F-−(CH4)n (n = 1−8) Anion Clusters

Infrared spectra of mass-selected F- -(CH4)n (n = 1-8) clusters are recorded in the CH stretching region (2500-3100 cm-1). Spectra for the n = 1-3 clusters are interpreted with the aid of ab initio calculations at the MP2/6-311++G(2df 2p) level, which suggest that the CH4 ligands bind to F- by equivalent, linear hydrogen bonds. Anharmonic frequencies for CH4 and F--CH4 are determined using the vibrational self-consistent field method with second-order perturbation theory correction. The n = 1 complex is predicted to have a C3v structure with a single CH group hydrogen bonded to F-. Its spectrum exhibits a parallel band associated with a stretching vibration of the hydrogen-bonded CH group that is red-shifted by 380 cm-1 from the nu1 band of free CH4 and a perpendicular band associated with the asymmetric stretching motion of the nonbonded CH groups, slightly red-shifted from the nu3 band of free CH4. As n increases, additional vibrational bands appear as a result of Fermi resonances between the hydrogen-bonded CH stretching vibrational mode and the 2nu4 overtone and nu2+nu4 combination levels of the methane solvent molecules. For clusters with n < or = 8, it appears that the CH4 molecules are accommodated in the first solvation shell, each being attached to the F- anion by equivalent hydrogen bonds.

Exploration of Basis Set Issues for Calculation of Intermolecular Interactions

The ab initio calculation of intermolecular interactions requires a large basis set to describe systems with dominant dispersion interaction accurately. This paper focuses on calculation of intermolecular bonding energies of weakly bound systems within the supermolecular method and on issues related to the choice of a basis set for these calculations, in particular size of the basis set, efficiency of 2-electron integral codes, basis set superposition error (BSSE), and the linear dependence of basis functions. In an attempt to find more efficient basis sets for calculations of intermolecular interactions, standard basis sets (10s Huzinaga, 6-311G**, cc-pV6Z), or their parts, are extended (tessellated) by a set of off-centered, s or p functions, symmetrically placed around the nuclei. Standard basis sets (10s Huzinaga, 6-311G**, cc-pVXZ, aug-cc-pVXZ, X = D, T, Q, 5, 6) are also augmented by sets of atom-centered, higher angular momentum functions (p, d, f). The distance from the nucleus of tessellating functions and orbital exponents of tessellating and augmenting functions are optimized with respect to the BSSE-corrected bonding energy at the MP2 or UCCSD level of theory. The two approaches are tested on the model systems with dominant dispersion interactions (3)H(2), (CH(4))(2), and Ne(2), and their efficiency is compared. Both tessellation and augmentation are successful in describing the intermolecular interactions of these model systems, with augmentation being more efficient. Our results draw attention to the linear dependence problems inevitably present in accurate calculations and confirm the need for underlying standard basis sets that provide good descriptions of core and valence electrons for the tessellation and augmentation approaches to be reliable.

Infrared Spectra and ab Initio Calculations for the Cl-−(CH4)n (n = 1−10) Anion Clusters

In an effort to elucidate their structures, mass-selected Cl--(CH4)n (n = 1-10) clusters are probed using infrared spectroscopy in the CH stretch region (2800-3100 cm(-1)). Accompanying ab initio calculations at the MP2/6-311++G(2df,2p) level for the n = 1-3 clusters suggest that methane molecules prefer to attach to the chloride anion by single linear H-bonds and sit adjacent to one another. These conclusions are supported by the agreement between experimental and calculated vibrational band frequencies and intensities. Infrared spectra in the CH stretch region for Cl--(CH4)n clusters containing up to ten CH4 ligands are remarkably simple, each being dominated by a single narrow peak associated with stretching motion of hydrogen-bonded CH groups. The observations are consistent with cluster structures in which at least ten equivalent methane molecules can be accommodated in the first solvation shell about a chloride anion.