Molecular dynamics simulations of fluid methane properties using ab initio intermolecular interaction potentials

Abstract

Intermolecular interaction energy data for the methane dimer have been calculated at a spectroscopic accuracy and employed to construct an ab initio potential energy surface (PES) for molecular dynamics (MD) simulations of fluid methane properties. The full potential curves of the methane dimer at 12 symmetric conformations were calculated by the supermolecule counterpoise-corrected second-order Møller-Plesset (MP2) perturbation theory. Single-point coupled cluster with single and double and perturbative triple excitations [CCSD(T)] calculations were also carried out to calibrate the MP2 potentials. We employed Pople's medium size basis sets [up to 6-311++G(3df, 3pd)] and Dunning's correlation consistent basis sets (cc-pVXZ and aug-cc-pVXZ, X = D, T, Q). For each conformer, the intermolecular carbon-carbon separation was sampled in a step 0.1 A for a range of 3-9 A, resulting in a total of 732 configuration points calculated. The MP2 binding curves display significant anisotropy with respect to the relative orientations of the dimer. The potential curves at the complete basis set (CBS) limit were estimated using well-established analytical extrapolation schemes. A 4-site potential model with sites located at the hydrogen atoms was used to fit the ab initio potential data. This model stems from a hydrogen-hydrogen repulsion mechanism to explain the stability of the dimer structure. MD simulations using the ab initio PES show quantitative agreements on both the atom-wise radial distribution functions and the self-diffusion coefficients over a wide range of experimental conditions.

A Refined Intermolecular Interaction Potential for Methane: Spectral Analysis and Molecular Dynamics Simulations

“…In our previous studies, the full methane pair potential energy surface has been calculated using both the wave-function based and density-functional based methodologies. [34][35][36] With the MP2/aug-cc-pVTZ level of theory we have constructed an ab initio force field and performed molecular dynamics simulations on fluid methane properties. 34 However, further applications revealed shortcomings of this force field, which have been attributed to the oversimplified model and the insufficiently precise ab initio data.…”

The structural investigation on small methane clusters described by two different potentials

“…Methane clusters are also essential since methane is one of the simplest hydrocarbon molecules. However, structural data of methane clusters have been limited as described below [19][20][21][22][23][24][25][26]. Hence the present study is aimed at theoretically investigating structures of methane clusters (CH 4 ) n with n ≤ 40.…”

“…Ab initio calculations on the methane dimer have been carried out [19][20][21][22][23]26]. Chao et al [23] reported ab initio potential energy curves for a variety of conformers of the dimer.…”

“…Chao et al [23] reported ab initio potential energy curves for a variety of conformers of the dimer. They adopted only a hydrogen-hydrogen Lennard-Jones potential function to produce the ab initio potential energy surface.…”

“…23 and 26, the value of r(C-H) was fixed. Chao and coworkers [22,23] showed that the optimization of the C-H bond length gives very small effect on the ab initio potentials. Hence the assumption that the structure of the methane molecule is rigid is reasonable in simulations of methane clusters.…”