Accurate ab initio calculations of methane dimer interaction energies and molecular dynamics simulation of fluid methane
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1177-Z09-21
Accurate ab initio calculations of methane dimer interaction energies and molecular dynamics simulation of fluid methane
Arvin Huang-Te Li and Sheng D. Chao Institute of Applied Mechanics, National Taiwan University, Taipei 106, Taiwan ROC., and Center for Quantum Science and Engineering, National Taiwan University, Taipei 106, Taiwan ROC.
ABSTRACT Intermolecular interaction potentials of the methane dimers have been calculated for 12 symmetric conformations using the Hartree-Fock (HF) self-consistent theory, the second-order Møller-Plesset (MP2) perturbation theory, the coupled-cluster with single and double and perturbative triple excitations (CCSD(T)) theory. The HF calculations yield unbound potentials largely due to the exchange-repulsion interaction. In MP2 and CCSD(T) calculations, the basis set effects on the repulsion exponent, the equilibrium bond length, the binding energy, and the asymptotic behavior of the calculated intermolecular potentials have been thoroughly studied. We have employed basis sets from the Slater-type orbitals fitted with Gaussian functions, Pople’s medium size basis sets to Dunning’s correlation consistent basis sets. With increasing basis size, the repulsion exponent and the equilibrium bond length converge at the 6-31G** basis set and the 6-311++G(2d, 2p) basis set, respectively, while a large basis set (aug-cc-pVTZ) is required to converge the binding energy at a chemical accuracy (~0.01 kcal/mol). We used the BSSE corrected results that systematically converge to the destined potential curve with increasing basis size. The binding energy calculated and the equilibrium bond length using the CCSD(T) method are close to the results at the basis set limit. For molecular dynamics simulation, 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 good agreement on both the atom-wise radial distribution functions and the self-diffusion coefficients over a wide range of experimental conditions.
INTRODUCTION The interaction potentials of hydrocarbons are crucial in determining the packing stability in solids and fluids and in biological soft matters. Methane is a prototype system of hydrocarbon interactions and thus has attracted intense theoretical studies on the interaction potentials of the methane dimer. These potentials are required in a molecular simulation to calculate bulk properties of fluids [1]. Most previous investigations of fluid methane properties used empirical force fields together with molecular dynamics (MD) or Monte Carlo simulations [2-3]. However, none of them can be universally applied to reproducing experiments quantitatively.
Recently, intermolecular potential energy surfaces (PESs) from first-principles calculations, or ab initio force fields, have been developed to simulate fluid methane properties. Their MD simulations reproduced some
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