Nonequilibrium molecular dynamics ͑NEMD͒ viscosity simulations of branched and linear alkanes at liquid densities were performed using both united-atom ͑UA͒ and all-atom ͑AA͒ intermolecular potential models in order to study the relative efficacy of the models in predicting fluid viscosity. Both models were used in conjunction with fixed bond lengths and bond angles, but different torsional potentials were investigated. The commonly used Ryckaert-Bellemans intermolecular potential model, which accurately predicts viscosities for short straight-chain alkanes, produced values for branched and long-chain alkanes that were significantly below experimental values. Likewise, a more complex UA model that uses transferrable site potentials and is commonly used to simulate thermodynamic properties also under predicted viscosities for branched and long-chain molecules. The UA models were also found to be density dependent, substantially under predicting viscosity at high liquid densities for all model fluids tested. Predicted viscosities using AA intermolecular potential models were generally substantially too large compared to experiment when using model parameters from the literature, even though thermodynamic properties were adequately predicted. However, evidence suggests that accurately modeling the hydrogen interactions and the rotation potential of methyl groups is essential for accurate viscosity simulations. Therefore, a new set of parameters for the hydrogen interactions was regressed using viscosity simulations of 2-methylpropane and n-pentane. Like the UA model, the AA model with the new parameters is still somewhat density dependent, but gives reasonably accurate predictions of viscosity for most fluids.

The NMR proton spin–lattice relaxation times T1 have been measured in the disordered crystalline phase of neopentane as a function of pressure and temperature. The molecular reorientation and self-diffusion contributions to the observed relaxation time are separated and the activation parameters are calculated for the two processes, respectively. The over-all molecular reorientation has a very weak pressure dependence whereas the self-diffusion exhibits a strong pressure dependence. The interesting result that the activation volume for self-diffusion is close to the molecular volume in neopentane is discussed in terms of the diffusion mechanism. The data enable us to estimate the modulated portion of the second moment attributable to the over-all molecular reorientation.