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Molecular dynamics simulations of methane molecules inside the (15,15) carbon nanotube (CNT) are performed for the temperature range from 173 to 293 K and pressures up to 700 bar. The structural and dynamic properties of 1-site and 5-site models of methane molecules are reported. The atomic model of the molecules increases density of methane in the vicinity of the nanotube wall, and the decrease of temperature increases the molecular density. The 5-site molecules from the contact layer exhibit tripod orientation with respect to the CNT. The diffusion coefficients of molecular translations along the carbon nanotube and rotational motion increase with temperature, and both decrease with pressure. Temperature dependences of the coefficients are described by the Arrhenius equation. Relatively free rotations of the 5-site molecules reduce the activation energies of translational diffusion compared to the energies for the 1-site molecules. The CNT flexibility, introduced by the reactive empirical bond order potential for interactions between carbon atoms of the nanotube, has weak impact on diffusivity of methane molecules. However, motions of the CNT atoms increase slightly the activation energies of the translational diffusion and diminish the energies of the rotational diffusion for higher pressures.

We report structural and dynamic properties of methane inside the (15,15) carbon nanotube (CNT) obtained from molecular dynamics simulations of flexible methane molecules, and intramolecular interactions are introduced by the reactive empirical bond order potential. The calculations are performed for a wide range of temperatures and loadings that correspond to states from the dense gas phase to the liquid state. The properties of flexible and rigid models of methane molecules are compared. The diffusivity of molecular translations along the CNT and rotations increase with temperature, and they decrease with pressure. Temperature dependences of the diffusion coefficients for flexible molecules are predicted by the Arrhenius equation. Internal motions of the CH4 atoms diminish the activation energies of the translational diffusion and increase the energies of the rotational diffusion, especially for higher pressures. The results mean that the possibility of changes of molecular bond lengths and valence angles in methane molecules causes a reduction of hindrances of their translations and at the same time it leads to the increase of rotational motion interruptions.